howl_guard: re-arm hold, soundcheck calibration, mandatory cap (phase 2 item 6b, PR C)

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PR C of the safety layer (phase 2 item 6b), stacked on #78 (feat/howl-guard, 55a62fd). Spec: the safety design note §4c, the decisions of 2026-10-01: (1) the re-arm hold, (2) a soundcheck calibration step, (3) cap_db mandatory for opening without a declaration.

What lands

• Re-arm hold (howl_guard.h). After a timer re-arm, the verdict trigger (LOST) arms only once the verdict has been continuously ok for release_hold_s (1.5 s, the same hold as a release). Before, the first ok block re-armed it. Walk releases and every other path are unchanged.
• Soundcheck sampler. calibrate_begin() samples each mic's D and A′ once per block for calibrate_s (30 s). calibrate_end(apply) returns a std::span<const guard_calibration>, one entry per mic: blocks, median / p95 / max of D and A′, and the suggested d_db / a_db = median + cal_d_margin_db (4 dB) / cal_a_margin_db (3 dB). With apply each mic runs on its own thresholds. set_thresholds(mic, d, a) restores a stored soundcheck; clear_calibration() returns to the policy's; threshold_d_db() / threshold_a_db() read them. Applied thresholds re-arm LOST (an ok block under them first), survive reset() and are not touched by set_policy().
  • Sampler: a fixed histogram, not a reservoir or a running quantile. 0.1 dB bins; D over [−60, +20) dB, A′ over [−100, +20) dB; 2000 uint32 counts (8 KB) per mic, allocated by the constructor. O(1) per block, exact to the bin width for any quantile, identical on every platform, no random source. A reservoir needs a random source and an O(n) selection; P² has no error bound on the bimodal window a cold start produces.
• Cap rule. PR B's ARMING already behaved this way without a cap: on timeout it stayed ARMING with unprotected raised and no OPEN. This PR documents the rule as mandatory and closes one hole: a mic that latched before any declaration (its floor was the cap) now re-arms when the cap is removed. Before, it kept the old cap as its floor.
• Tests. State machine (float and double, both emulated selections): the re-arm hold (one block short of the hold does not arm LOST; the full hold does; a walk release still does), the sampler (exact bins, percentiles, the window cut-off, apply / reset / clear / out-of-range), the no-cap rule and the latched-undeclared re-arm, noexcept on the new entry points. Host:
  • three new gated rows: ColdStartWithoutACapStaysArmed, SoundcheckCalibrationOnStableMaterial and SoundcheckCalibrationSeesAWalk;
  • LouderCouplingRearms and ColdStartWithoutBackingTrack now gate the new numbers;
  • two new MUTAP_SLOW sweeps, CalibrationMargins (shadow guards over a 9 × 8 margin grid) and CalibrationApplied;
  • LouderCoupling is split out of WalksAndLouderCoupling, which is now Walks.
• docs/howl-guard.md (new sections, every number from this PR's runs), HANDOFF item 12, the header comments, a README line.

Measured (macOS 15.7 x86_64, i9-8950HK, AppleClang 17, Release, double; every run from reset)

1. Re-arm hold: F → 2F before → after

"Before" is this PR's test harness on PR B's guard (55a62fd). Same rows, same host.

	Runs	Re-armed	Re-arm after the duck, median [min, max] (s)	LOST-ducks after the first re-arm (runs)	All LOST-ducks after the change	Howl blocks after the change / after the re-arm
Gated (cabin, mt5, studio, hall × seeds 1, 21), before	8	6	5.00 [5.00, 10.01]	16 (6)	28	0 / 0
Gated, after	8	6	5.00 [5.00, 10.01]	0 (0)	12	0 / 0
Sweep (six rooms × five seed sets), before	30	26	5.00 [5.00, 16.73]	47 (20)	87	0 / 0
Sweep, after	30	26	5.00 [5.00, 16.73]	1 (1)	41	0 / 0

Time ducked or releasing after the change, median per room, before → after: gated mt5 0.96 → 0.51, studio 0.94 → 0.26, hall 0.93 → 0.26, cabin 0.56 → 0.56; sweep mt9 0.95 → 0.77.

• Gated as a rate with margin: at most 4 LOST-ducks after a re-arm over the 8 runs (0.5 a run, a quarter of the old 2.0), and at least 4 of 8 re-armed. Not a bare zero.
• Cabin still cycles, by another route. Its verdict holds ok while ducked, so it releases on the walk path, not by a timer re-arm. The verdict is then lost again at full gain: 7 LOST-ducks in 2 gated runs, 18 in 5 sweep runs, unchanged. The sweep's one remaining LOST-duck after a re-arm is also cabin's. A LOST within probation counted as a strike would reach this case; it was not measured.
• No regression: the walk rows (gated and sweep) and every cold-start row that PR B printed read the same to the last printed digit.
  • Walk at exact − 6: release − reconvergence median 1.59 s [1.38] gated, 1.60 s [1.38] sweep; 0 early.
  • Walk at the limit − 6: 0 of 12 / 0 of 30 ducked.
  • Declaration medians: 1.77–2.07 s gated, 1.76–2.11 s sweep.
  • Gap, two-mic, audible-cost and bus-stage gated tables: identical as well.

2. Soundcheck calibration: the margin sweep

HowlGuardSweep.CalibrationMargins: 210 runs at the canceller's limit − 6 with the backing track, a 30 s soundcheck from reset.

• Stable runs (180): 30 s more of the same song; the audible-cost grid (six rooms × five seed sets × voiced+aux / speech+aux × 0 / 2 / 5 Hz).
• Walk runs (30): the six walks × five seed sets, the walk at 40 s.
• Method: the live guard keeps the factory thresholds. Beside it, 72 shadow guards (one per margin pair) take the same residual and statistics and apply their soundcheck at 30 s. A shadow's first duck is exact until its gain leaves the live guard's; 0 inexact here.

Stable runs with a duck after the soundcheck, of 180:

D margin \ A′ margin	+ 0 dB	+ 1 to + 10 dB, or A′ off
+ 0	174	140
+ 0.5	135	36
+ 1	131	11
+ 1.5	131	7
+ 2	131	3
+ 3	131	0
+ 4 (default)	131	0
+ 6	131	0
D off	131	0

Walks seen, of 30 (a duck within 5 s of the change); in brackets, runs that ducked before the walk:

D margin \ A′ margin	+ 0	+ 1	+ 2	+ 3 (default)	+ 4 to + 10, or off
+ 0	0 [30]	2 [28]	2 [28]	2 [28]	2 [28]
+ 0.5	0 [30]	13 [17]	13 [17]	13 [17]	13 [17]
+ 1	0 [30]	21 [9]	21 [9]	21 [9]	21 [9]
+ 1.5	0 [30]	24 [6]	24 [6]	24 [6]	24 [6]
+ 2	0 [30]	28 [2]	28 [2]	28 [2]	28 [2]
+ 3	0 [30]	30 [0]	30 [0]	30 [0]	30 [0]
+ 4 (default)	0 [30]	30 [0]	30 [0]	30 [0]	30 [0]
+ 6	0 [30]	28 [0]	18 [0]	16 [0]	16 [0]
D off	0 [30]	23 [0]	1 [0]	1 [0]	1 [0]

2079.35 s on 4 threads.

The margins.

• D: the smallest margin with 0 stable ducks is + 3 dB (+ 2 dB: 3 runs ducked). The default is + 4 dB: one step of headroom, 2 dB over the last margin that ducked.
• A′: + 0 always ducks, since half the blocks sit above the median. + 1 dB already ducked none. The default is + 3 dB, a factor of 2.
• Walks at the defaults: 30 of 30 seen, 0.31 s after the change. The factory thresholds saw 0 of 30.

The soundcheck medians over the 210 runs:

• D: −12.15 to −6.25 dB, 5 to 11 dB under the factory −1.235.
• A′: −30.55 to −25.85 dB.

What the soundcheck prints (gated row; two of its eight lines):

  cabin voiced+aux seed 1            soundcheck 22500 blocks: D median   -8.15 dB (p95   -5.55, max   +0.15); A' median  -27.75 dB (p95  -27.15, max   -0.05) -> d_db   -4.15, a_db  -24.75 (applied)
  mt5 speech+aux seed 1              soundcheck 22500 blocks: D median   -8.35 dB (p95   -7.35, max   +0.45); A' median  -29.15 dB (p95  -25.45, max   -0.05) -> d_db   -4.35, a_db  -26.15 (applied)

Applied (the live guard on its soundcheck thresholds at the default margins):

	Stable runs with a duck	Walks ducked (by LOST)	Duck after the walk, median (s)	Release − misalignment-oracle reconvergence, median [min] (s)	Early	Howl blocks
Gated (cabin, mt5; seeds 1, 21)	0 of 8	12 of 12 (11)	0.31	1.74 [1.63]	0	0
Sweep (CalibrationApplied, 1227 s)	0 of 180	30 of 30 (29)	0.31	1.74 [1.63]	0	0

Applied d_db across the sweep: −8.15 to −2.25 dB; a_db: −27.55 to −22.85 dB. The duck not by LOST was a detector TRIP (mt5 → mt105).

Gates, with margin:

• stable: at most 2 of 8 runs with a duck (measured 0);
• walks: at least 8 of 12 ducked (measured 12), the release median over 0.5 s (1.74), at most 2 early (0);
• 0 howl blocks;
• the shadow at the live margins tracks the live guard exactly (structural).

3. Cap rule: track-off rows

20 s from reset at the limit − 6 (gated: cabin and mt5, seeds 1, 21, 41).

Row	Cap	Runs	Declared	OPEN_CAPPED	Time to OPEN_CAPPED, median [max] (s)	unprotected from (s)	Time in ARMING, median (s)	Ducks after OPEN_CAPPED (runs)	Howl blocks
held note	dry limit − 6	6	0	6	10.00 [10.00]	10.00	10.00	2 (2)	0
music	dry limit − 6	6	0	6	10.00 [10.00]	10.00	10.00	0	0
speech	dry limit − 6	6	0	6	10.00 [10.00]	10.00	10.00	0	0
held note	none	6	0	0	—	10.00	20.00 (all)	0	0
music	none	6	0	0	—	10.00	20.00 (all)	0	0
speech	none	6	0	0	—	10.00	20.00 (all)	0	0

"10.00" is block 7499, 9.9987 s.

Sweep (six rooms × five seed sets, 30 runs a row):

• Under the cap: every track-off run reached OPEN_CAPPED at 10.00 s, with 0 howl blocks. The held note ducked after OPEN_CAPPED in 11 of 30 runs at S1 (9 of 20 at exact + 3, 4 of 20 at exact + 6), music in 1 of 20 at exact + 3, speech never. These are detector TRIPs (LOST is disarmed before a declaration), not classified against the burst oracle.
• Without a cap (held note, music, speech): 90 of 90 runs stayed in ARMING for all 20 s, 0 howl blocks.
• The guarded cold-start sweep now totals 690 runs with 0 howl blocks.

Defaults

None of PR B's defaults moved. New fields:

• calibrate_s = 30: the protocol's window.
• cal_d_margin_db = 4 and cal_a_margin_db = 3: measured above.

What did not separate, or could not be measured

• F → 2F through the walk path (cabin) still cycles; the LOST-in-probation-as-a-strike alternative was not measured.
• The margins are one loop's and one song's. Not measured:
  • a soundcheck on one song with the show on another;
  • a soundcheck at a different gain from the show;
  • a soundcheck without the backing track: the verdict never declares there, and the sampler reads ARMING's statistics.
• The 30 s window includes about 2 s of the cold start before the declaration.
• Under the cap, the held note's post-OPEN_CAPPED ducks were not classified against the burst oracle.
• arm64 / Linux numbers: only CI pass / fail. The new gates leave room for platform drift, but no second host has printed these tables.
• HowlGuardSweep.OperatingPoints was not re-run: the guard is not in it.

Verification

• cmake -S . -B build -G Ninja -DCMAKE_BUILD_TYPE=Release -DMUTAP_WERROR=ON && cmake --build build: clean.
• ctest --test-dir build --output-on-failure: 374 of 374 passed (21 skipped: the 20 MUTAP_SLOW sweeps and PortableRandom.MatchesLibstdcxxBitForBit), 3143.71 s. The 16 gated guard rows took 53.09 to 88.65 s each (a first full run failed ColdStartWithoutBackingTrack on my own bound, ≥ 10.0 s against block 7499 = 9.9987 s; fixed to 10 − 1.5 blocks, then the full run above).
• Gated rows alone (--gtest_filter='HowlGuardHost.*'): 777.37 s.
• Sweeps, 4 threads (MUTAP_SLOW=1 MUTAP_SLOW_THREADS=4): CalibrationMargins 2079.35 s, CalibrationApplied 1227 s, ColdStart 1890 s, Walks 245 s, LouderCoupling 88 s, TwoMicsAndAudibleCost 567 s (its two-mic and audible-cost tables match PR B's).
• afc_chain.h is unchanged (bit-identical without a guard by construction). mutap_fingerprint passed. No icount or fingerprint workload includes afc_chain.h or howl_guard.h.
• pre-commit run --files <changed>: passed. scripts/tidy.sh on test_howl_guard.cpp, test_howl_guard_host.cpp and test_afc_chain.cpp: clean. A direct clang-tidy-18 -p build-tidy count on each: 0 warnings.
• GCC safety: the new trace loop in the host test walks clamped_range iterators, as 55a62fd does. The calibrate_end / quantile loops walk iterators over each mic's histogram slice.

Follow-up commit 8c2e368 (CI)

• HowlGuardHost.CostPerBlock: Linux GCC CI (job 110465810942) read the guard at 4.61 / 6.86 % (float, 1 / 2 mics) and 4.59 / 7.15 % (double) of a canceller, against 0.57 / 1.09 / 0.84 / 1.73 % on the Intel Mac, failing the 4.5 / 7 % bounds by 0.001. The ratios are now printed and RecordProperty'd; the gate is < 25 % (a gross regression only).
• HowlGuardHost.ColdStartWithoutBackingTrack: macOS arm64 CI ducked after OPEN_CAPPED in 5 of 6 held-note runs (Intel 2 of 6) and failed duck_runs <= 4. The ducks are now classified against the burst oracle: on Intel both are within 0.5 s of a +20 dB loop-born burst (0 off a burst). Gated: 0 howl blocks, OPEN_CAPPED at the timeout (>= 10 s − 1.5 blocks, median < 11 s), runs with a duck off any burst <= 6 of 18 (Intel 0). The duck count is reported, not gated.
• Re-run locally after the change: both rows pass (52.17 s, 17.69 s); pre-commit and scripts/tidy.sh clean, a direct clang-tidy-18 count 0.

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