fix(engine security): close witness domain-separation collision in governed-trust cycle + prove privacy monotonicity (#1074)

* fix(engine): length-prefix witness fields to close domain-separation collision

The BLAKE3 trust witness concatenated model_version, calibration_version,
and privacy_decision boundary-to-boundary, with the variable-length evidence
list lacking an explicit count. A string straddling a field boundary (e.g. a
per-room adapter id absorbing the leading bytes of the calibration epoch, or a
model_version absorbing a trailing evidence ref) collided with a different
trust decision — silently un-distinguishing two distinct privacy-relevant
inputs and defeating the ADR-137 tamper/drift audit guarantee. model_version
is operator-influenceable via the adapter id (ADR-150 §3.4), so the ambiguity
was reachable.

Fix: domain-tag the hash and length-prefix every field (8-byte LE length),
plus an explicit evidence count. Pinned by two fails-on-old tests:
witness_distinguishes_model_calibration_boundary and
witness_distinguishes_evidence_model_boundary.

Co-Authored-By: claude-flow <ruv@ruv.net>

* test(engine): pin privacy monotonicity, fail-closed boundaries; de-magic constants

Review hardening for the governed-trust cycle (no behavior change):

- forced_contradiction_never_relaxes_class: property test over all 5 privacy
  modes proving a forced contradiction only ever raises the emitted class byte
  (more restrictive) and a clean cycle emits exactly the base class — the
  ADR-141/120 information-only-removed invariant.
- empty_cycle_fails_closed: a zero-frame cycle errors (fusion NoFrames),
  emits no SemanticState, and does not advance the cycle counter.
- single_node_cycle_is_well_formed: characterizes the n=1 boundary (no mesh,
  no directional, base class, witness still emitted) — documents single-node
  sensing as a valid non-demoting mode, not a bypass.
- De-magicked the engine-construction literals (coherence accept gate, ADR-143
  SLAM discovery + static-anchor thresholds) into named documented consts,
  value-identical, pinned by engine_constants_match_prior_values.

Co-Authored-By: claude-flow <ruv@ruv.net>

* docs(engine-review): record witness domain-separation fix + monotonicity clean bill

CHANGELOG [Unreleased] Security entry and review notes appended to ADR-137
(witness domain-separation fix) and ADR-141 (privacy monotonicity confirmed
clean over all 5 modes, fail-closed boundaries pinned).

Co-Authored-By: claude-flow <ruv@ruv.net>

CHANGELOG.md

@@ -19,6 +19,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - **ADR-260: RuField MFS — the open specification for camera-free multimodal field sensing.** A common event / tensor / calibration / privacy / provenance model that sits *above* WiFi CSI/CIR/BFLD, UWB, BLE Channel Sounding, mmWave radar, ultrasound, subsonic, infrared, and future quantum sensors (each modality emits a normalized `FieldEvent` → `FieldTensor` → `FusionGraph` → `PrivacyClass` → `ProvenanceReceipt`). Published as a **standalone repo** [`ruvnet/rufield`](https://github.com/ruvnet/rufield) and vendored here as the `vendor/rufield` submodule (the `vendor/rvcsi` pattern — not a `v2/` workspace member). The v0.1 reference stack is a self-contained 6-crate Rust workspace (`rufield-core`, `-provenance` [sha256 + ed25519], `-privacy` [P0–P5 guard], `-adapters` [deterministic `SyntheticSim` across wifi_csi/mmwave_radar/infrared_thermal], `-fusion` [graph + TOML weighted-Bayes rules → 7 room-state inferences], `-bench` [deterministic runner + the §31 acceptance test]). **60 tests / 0 failed, clippy-clean.** §27 acceptance criteria 1–8 and 10 PASS; the live dashboard (9) is deferred. **All benchmark metrics are SYNTHETIC** (scored against the simulator's own ground truth — presence/breathing/bed_exit/room_transition F1 = 1.000, nocturnal_scratch 0.923 reported honestly, p95 latency ~0.01 ms, provenance coverage 100%, 0 privacy violations) — they prove the pipeline recovers known truth, **not** field accuracy; real hardware adapters (ESP32 CSI, mmWave, thermal IR) are a documented roadmap item, none validated in v0.1. The Python deterministic proof is unchanged (rufield is off the signal-processing proof path).
 
 ### Security
+- **`wifi-densepose-engine` governed-trust review — witness domain-separation gap FIXED + privacy monotonicity confirmed clean (ADR-137 / ADR-141 / ADR-032).** Beyond-SOTA correctness+security review of the security-critical composition root (the cycle enforcing RuView's privacy guarantees), not covered by the ADR-154–159 sweep. **One real witness-integrity bug fixed.** `witness_of` concatenated `model_version`, `calibration_version`, and `privacy_decision` boundary-to-boundary and left the variable-length evidence list without a count, so a string straddling a field boundary collided with a *different* trust decision — e.g. a per-room adapter id (ADR-150 §3.4, operator-influenceable) absorbing the leading bytes of the calibration epoch (`model="…cal:00a"`,`cal="b"`) yields the same witness as `model="…"`,`cal="cal:00ab"`. Two distinct privacy-relevant input tuples → one witness defeats the ADR-137 §2.7 "any privacy-relevant delta → different witness" tamper/drift audit. **Fix:** domain-tag the BLAKE3 hash (`ruview.engine.witness.v1`), write an explicit evidence count, and **length-prefix every field** (8-byte LE length ‖ bytes) — unambiguous framing regardless of contents. Witness-layout change by design (prior witness bytes invalidated); downstream consumers (`engine_bridge`, rufield) assert only witness *relationships* (`assert_ne`/`assert_eq` across runs), never absolute bytes, so nothing breaks. Pinned by two fails-on-old tests: `witness_distinguishes_model_calibration_boundary`, `witness_distinguishes_evidence_model_boundary`. **Dimensions confirmed clean (with evidence, no invented issues):** (1) **privacy monotonicity** — `effective_class` is recomputed each cycle from the active mode's floor with at most a single-step `demote_one` (clamped at `Restricted`), no cross-cycle state, proven over **all 5 modes** by `forced_contradiction_never_relaxes_class` (forced contradiction only ever raises the class byte; clean cycle == base); (2) **fail-closed** — empty cycle errors with no degenerate output (`empty_cycle_fails_closed`), single-node boundary characterized (`single_node_cycle_is_well_formed`), NaN coupling → `max(0.0)`→absent edge→at-risk (more restrictive); (3) **witness determinism** — no HashMap iteration / float formatting feeds the hash; (4) **mesh_guard** (ADR-032) — partition-risk → demotion path verified, thresholds already named documented fields. De-magicked the engine-construction literals (coherence accept gate, ADR-143 SLAM discovery + static-anchor thresholds) into named documented consts, value-identical, pinned by `engine_constants_match_prior_values`. `wifi-densepose-engine --no-default-features`: **27→33 tests**, 0 failed (+2 witness, +1 monotonicity property, +2 fail-closed boundary, +1 de-magic pin). Python deterministic proof unchanged (`f8e76f21…46f7a`, bit-exact — the engine is off the signal proof path). Review notes appended to ADR-137 (witness) and ADR-141 (monotonicity).
 - **ADR-157 Milestone-1 B4 - constant-time HMAC sync-beacon tag compare (`wifi-densepose-hardware`).** `AuthenticatedBeacon::verify` compared the 8-byte HMAC-SHA256 tag with `self.hmac_tag == expected`, which short-circuits on the first differing byte and leaks, through verification latency, how many leading bytes an attacker's forged tag matched - a byte-by-byte tag-recovery oracle (~256*N trials instead of 256^N). Replaced with a hand-rolled branch-free `constant_time_tag_eq` (XOR-accumulate every byte difference into a single `u8`, no early exit, `#[inline(never)]` + `core::hint::black_box` to stop the optimizer reintroducing a short-circuit or a non-constant-time `memcmp`). **No new dependency** - ADR-157 had deferred this only to avoid adding the `subtle` crate; a fixed 8-byte compare needs none. Grade MEASURED (constant-time *construction*; micro-timing on a noisy host is a smoke check only, gated `#[ignore]`). Pinned by `tag_compare_is_constant_time_shape` (equal/first-differ/last-differ/all-differ/length-mismatch + an end-to-end `verify()` last-byte tamper), proven to fail on a last-byte-skipping constant-time bug. ADR-157 §8 B4 -> RESOLVED.
 - **ADR-080 open HIGH findings closed on the Rust `wifi-densepose-sensing-server` boundary (ADR-164 G11).** The QE sweep's three HIGH findings — XFF-spoofing bypass, leaked stack traces, JWT-in-URL (CWE-598) — were logged against the Python v1 API and never re-verified against the shipped Rust sensing-server; the HOMECORE/M7 sweep (ADR-161) covered `homecore-server`, not this crate.
   - **#2 leaked internal errors (the one live exposure) — FIXED.** Six handlers in `main.rs` serialized the internal error `Display` straight into the JSON response body: `edge_registry_endpoint` returned a panicked `spawn_blocking` `JoinError` (`"task … panicked"`) in a `500`, plus the raw upstream error in a `503`; `delete_model`/`delete_recording`/`start_recording` returned `std::io::Error` strings (OS detail / path); `calibration_start`/`calibration_stop` returned the `FieldModel` error chain. New `error_response` module logs the full detail **server-side only** (with a correlation id) and returns a generic body (`{"error":"internal_error","correlation_id":…}`) — no `panicked`, no file paths, no Debug chain. 5 module tests (a leak-substring guard proven to fail on the reverted old body) + the existing handler suite.