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>

docs/adr/ADR-137-fusion-engine-quality-scoring-evidence.md

@@ -495,3 +495,34 @@ Rejected. `ViewpointFusionEvent` (viewpoint/fusion.rs lines 183–219) is an int
 **Integration glue -- not yet on the live path:** emission of `CalibrationIdMismatch` / `DriftProfileConflict` / `PhaseAlignmentFailed` once `calibration_id` propagation and the phase-align convergence signal are threaded onto frames; the BFLD witness record emitted on privacy demotion.
 
 **Trust contribution:** sensor *agreement made explicit* -- fusion records the evidence it relied on, and any disagreement automatically tightens the downstream privacy class.
+
+---
+
+## Witness Integrity Review (2026-06-14) — domain-separation fix
+
+A beyond-SOTA security review of `wifi-densepose-engine` (the composition root
+that builds the §2.7 trust witness in `witness_of`) found a real **witness
+domain-separation gap**, now fixed.
+
+**Finding (witness-gap, HIGH).** `witness_of` concatenated `model_version`,
+`calibration_version`, and `privacy_decision` boundary-to-boundary, and the
+variable-length `evidence` list carried no explicit count. A string straddling a
+field boundary therefore collided with a *different* trust decision —
+e.g. a per-room adapter id (ADR-150 §3.4, operator-influenceable) that absorbs
+the leading bytes of the calibration epoch (`model="…cal:00a"`, `cal="b"`)
+produces the **same** witness as `model="…"`, `cal="cal:00ab"`. Two distinct
+privacy-relevant input tuples → one witness defeats the "any privacy-relevant
+delta → different witness" guarantee this ADR's §2.7 witness exists to provide.
+
+**Fix.** The witness now (a) prepends a domain tag `ruview.engine.witness.v1`,
+(b) writes an explicit 8-byte evidence count, and (c) **length-prefixes every
+field** (8-byte LE length ‖ bytes), so field framing is unambiguous regardless
+of contents. This is a witness-layout change (all prior witness bytes are
+invalidated by design); downstream consumers only assert witness *relationships*
+(`assert_ne`/`assert_eq` across runs), not absolute bytes, so nothing breaks.
+
+Pinned by `witness_distinguishes_model_calibration_boundary` and
+`witness_distinguishes_evidence_model_boundary` (both fail on the old
+concatenation). Witness **determinism** was reviewed and confirmed clean: no
+HashMap iteration and no float formatting feed the hash (floats appear only in
+the `SemanticState` statement, which is outside the witness).

docs/adr/ADR-141-bfld-privacy-control-plane-modes-attestation.md

@@ -599,3 +599,33 @@ Per ADR-028/ADR-010, three rows are added to the witness log:
 **Integration glue -- not yet on the live path:** wiring the registry into `PrivacyGate` class transitions, the MQTT discovery payload, and a read-only Home Assistant diagnostic entity exposing the active mode + proof hash.
 
 **Trust contribution:** the *policy spine* -- privacy posture is a tamper-evident, auditable chain rather than a checkbox; an operator's mode choice actively governs whether identity data may even exist.
+
+---
+
+## Privacy Monotonicity Review (2026-06-14) — confirmed clean
+
+A beyond-SOTA security review of the governed-trust cycle
+(`wifi-densepose-engine::StreamingEngine::process_cycle_calibrated`) examined
+the privacy-demotion path this ADR governs. **The monotonicity invariant holds:
+demotion only ever makes the emitted class more restrictive, never less.**
+
+Verification (no behaviour change, the result is a clean bill with evidence):
+
+- Each cycle computes `effective_class` fresh from the active mode's
+  `target_class()` (the floor) and applies at most a **single-step** demotion
+  (`demote_one`, clamped at `Restricted`). There is no cross-cycle state that
+  could let a permissive class overwrite a restrictive one.
+- A forced contradiction (calibration mismatch / array-geometry insufficiency /
+  mesh partition risk, ADR-032) raises the class byte; a clean cycle emits
+  exactly the base class.
+- Pinned by `forced_contradiction_never_relaxes_class`, a property test over
+  **all five** `PrivacyMode`s asserting `effective_class.as_u8() >=
+  base_class.as_u8()` (strictly greater unless already clamped at `Restricted`)
+  under a forced contradiction, and `== base` on a clean cycle.
+
+Fail-closed boundaries were also pinned: an empty cycle errors (no degenerate
+over-permissive output, `empty_cycle_fails_closed`) and the single-node boundary
+is characterized as a valid non-demoting mode (`single_node_cycle_is_well_formed`).
+
+The related witness domain-separation fix from the same review is recorded in
+ADR-137 (the witness folds `effective_class`, so the demotion is auditable).