claw: reel_still.t27 -- the TRI CLAW /mcp tool that renders one particle still

[gHashTag]

Closes #10547

Stacked on #10510 (draft, branch claude/visual-output-10508): this spec imports STILL_MS and still_ms_ok from specs/claw/visual_output.t27, which exists only there. Draft until #10510 lands; then retarget to master.

What

specs/claw/reel_still.t27 (module ClawReelStill) is the contract for reel_still, the TRI CLAW /mcp tool that specs/claw/visual_output.t27 names as its follow-up. It takes topic, lesson, lang and aspect and returns one image/png: the frame a token_floor reel shows at STILL_MS.

• Name: reel_still has no t27_/t27f_ prefix, so specs/t27mcp/service.t27 homes it in TRI CLAW and mounts it as it is. A control checks that t27f_reel_still would be refused.
• Topic: "" or token-floor is the physics reel. A lesson id resolves through specs/t27f/reel_topics.t27, code point by code point, so the tool and the reels share one list of lessons. A lesson index alone names the same reel, and topic and lesson must agree.
• Frame: STILL_MS * SCENE_FPS / 1000 = 270 of 450. The renderer env is TF_SHARD=270 TF_SHARDS=450 TF_CUT=<aspect>, which frame_shard.first_of/end_of turn into exactly [270, 271). A control checks that a halved shard count renders the whole reel.
• Overlays: they come from the reel's own filter graph, whose drawtext overlays are keyed on the filter frame index n, not on time. A lone frame is n = 0 however it is offset, so -itsoffset 9 drew the frame-0 overlays (measured: 42.9 dB against the raw particle frame, 23.3 dB against the reel). The recipe puts tpad=start=270 in front of [0:v], so the frame arrives as n = 270 at 9 s; the last [v] frame goes to the PNG (-update 1), [a] to null. Measured on the lab, lesson 1 en tall: rc 0, 2585 ms, 499707 bytes, 49.3 dB against frame 270 of the reel's mp4 (21.7 dB against 269, 22.0 dB against 271).
• Lab only: off the lab the verdict is V_NOT_ON_LAB, and nothing is rendered.

Evidence (Railway lab)

• t27c test report: 15 tests, 15 pass, 0 vacuous.
• t27b (aarch64 under qemu): ClawReelStill: 15 passed, 0 failed.
• Mutants:
  • topic lookup returns 0: 3 tests fail.
  • frame formula /1001: 1 fails. /999 is an equivalent mutant, because 270000/999 still truncates to 270.
  • lab check removed: 1 fails.
  • pad one frame short (still_frame() - 1): 1 fails.
• Seal .trinity/seals/claw_ClawReelStill.json: 15/15, vacuous 0. Its closure is visual_output, frame_shard, reel_topics, token_floor_scenes and t27mcp/service.
• t27b ledger row pass; one row in docs/claw/README.md.

Not in this PR

Serving the tool from TRI CLAW's /mcp (Railway glue) is not here. The header of visual_output.t27 still says no tool renders a still, which stays true until that glue exists.

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