spec(port): port the cover-traffic payload size variance guard (Refs #8887)

[dmitrii-f-t27]

Refs #8887 (CR-CHAT-07 ring tracker). Parent epic: gHashTag/trios#1812 (W2, own lane, T-PORT). Fourteenth CR-CHAT-07 ring target delivered: the size-variance rules (R-CHAT-10) — batch ceiling (1024), per-observation zero-timestamp probe, strictly-increasing timestamps, size floor (256) / ceiling (4096; band-edge pair accepted structurally, rejected statistically), then over ≥ 2 observations the CV ceiling (0.2) — D5 verdict with every payload field (prev/current ride the verdict for NonMonotonic), original rule order, all ten CPSV tests transcribed plus a rule-order test (11 total, 0 vacuous). compute_cv (float mean + sqrt of usize sums; int→float casts do not lower) arrives caller-computed per the lane convention (#9698/#9747/#9811/#9814); structural rules fully in-spec; all fixture CVs Python-verified exact (band-edge 0.8824, uniform 0.0, slight 0.0148). t27b 11/11 native; t27c 11/11; gen-rust compiles under rustc (0 errors, 0 &'static str, 0 Vec<const); coverage Untested 0; seal all hashes MATCH; ledger row included (pass, non-pass untouched). Dedup checked. Sequencing: one-line ledger rebase if the earlier lane PRs land first — I take it on request.

[dmitrii-f-t27]

Merge request for the gHashTag merger: steward READY (required all SUCCESS — validate, check-linked-issue, parse-ratchet, t27b-native-linux, t27b-native-macos; the red ones are non-required per Q16 and red on master too). Ledger row included; one-line rebase if the earlier lane PRs land first — I take it on request.

[gHashTag]

Carried unchanged, with its original author, as one commit of #10097 (tracking #10096). That PR lands all 25 green CR-CHAT-04/07 ports at once, because each single PR kept conflicting on the ledger rows before its CI could finish. Closing this one so its queued jobs free the Actions queue.