[dmitrii-f-t27]

The twenty-ninth CR-CHAT-07 ring target (W2, R-CHAT-10, tracker #8887): port of crates/trios-chat/rings/CR-CHAT-07/src/observer_fingerprint_rotation_guard.rs.

A wire observer fingerprints a device by its stable characteristics (cipher suite, ALPN, burst gap, size class); an unchanged fingerprint tracks the user across sessions, links chat traffic to other protocols and re-identifies after a gap. The rules, in the original's order:

• the tracked ceiling (256),
• the empty batch accepted,
• per session: the empty-fingerprint probe, the length ceiling (128),
• the run fold: a session equal to its predecessor grows the same-fingerprint run, a run past the window of eight answers NoRotation with the run length, a change counts one rotation,
• the rotation ceiling (64) over the whole batch.

Carried as the D5 verdict (Ok-first enum + named struct with the one payload field — the run length for NoRotation). The original's SameAsPrevious is declared and never answered; it stays in the enum, declared and unreachable exactly as in the original.

Adaptations (t27 conventions): Vec<u8> fingerprint → fixed [128]u8 capacity + fp_len; Option<&[u8]> previous → has_prev flag over the record fields; fingerprints compare over the shared length via a whole-form scan helper with direct tests.

All 10 original OFRG tests transcribed with exact asserts, plus the helper test and a rule-order test (12 total): the tracked ceiling answers over a flooded batch of empty forms; the per-session probes answer inside the loop before the rotation ceiling is read (a sixty-fifth rotation waits while the last session answers its own empty or over-long code); the run fold answers NoRotation at nine.

Checks (conventions 0-14): t27b test 12 passed 0 failed, 0 blockers; t27c test-report 12/12, 0 vacuous; gen-rust output compiles under rustc (0 'static str, 0 Vec<const); coverage Untested 0; ASCII only; seal saved, all hashes MATCH; ledger row added to docs/reports/t27b_expectations.json.

Refs #8887

[dmitrii-f-t27]

All required checks green (validate, check-linked-issue, parse-ratchet, t27b-native-linux, t27b-native-macos); cli-tri / duplicate-bodies / t27b-native-ratchet are the known non-required reds. Steward READY. Requesting owner merge when convenient — 29 of ~34 CR-CHAT-07 targets delivered, Refs #8887.