cog-ha-matter (ADR-116 P4): pure witness hash-chain primitive

Second P4 unit: an append-only SHA-256 hash chain for tamper-evident
audit logging. ADR-116 §2.2 promised this for healthcare /
education / shared-housing deployments — this lands the primitive
with no key dependency so the next iter can layer Ed25519 signing
on top without touching the chain itself.

Module shape:

  * `WitnessHash([u8; 32])` newtype + `WitnessHash::GENESIS` sentinel
  * `WitnessEvent { seq, prev_hash, ts, kind, payload, this_hash }`
    — once committed, every field is immutable
  * `WitnessChain` — `append`, `tip`, `verify`, `events`
  * `canonical_bytes` — length-prefixed serialization that prevents
    the classic concatenation forgery
    (`abc|def` ≠ `ab|cdef`)
  * `WitnessVerifyError` — auditor-friendly error with `at: usize`
    on every variant (SeqGap, PrevHashMismatch, HashMismatch)

13 new tests covering both happy path and active tampering:

  * genesis hash all-zeros
  * empty chain tip is genesis
  * canonical bytes length-prefixed (anti-forgery)
  * canonical bytes start with prev_hash (wire-format lock)
  * append links to prev_hash
  * seq monotonic from 0
  * verify passes on clean chain
  * verify catches tampered payload (fires HashMismatch)
  * verify catches broken prev_hash link
  * verify catches seq gap
  * hash hex is 64 lowercase chars
  * first event prev_hash == GENESIS (auditor anchor)
  * different payloads → different hashes

Hash-chain over Merkle is the right tradeoff for the cog's event
rate (a few/min steady, dozens during a fall) — linear scan is
fine and we save the Merkle complexity for a future tier when
chains span days.

34/34 cog tests green (21 → 34).

ADR-116 P4 row updated to enumerate the three P4 sub-units shipped /
pending: (a) mDNS record-builder ✅, (b) witness hash-chain ✅, (c)
responder + embedded broker + Ed25519 signing pending.

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

docs/adr/ADR-116-cog-ha-matter-seed.md

@@ -95,7 +95,7 @@ Ranked by build cost × user impact:
 | **P1** | Research dossier ([`docs/research/ADR-116-ha-matter-cog-research.md`](../research/ADR-116-ha-matter-cog-research.md)) | ✅ **done** — 8 sections, 30+ citations, v1 scope ranked |
 | **P2** | Cog crate scaffold (`v2/crates/cog-ha-matter/`) — Cargo.toml + `src/{lib,main,manifest}.rs`, workspace member, CLI args, `--print-manifest` flag, 2 manifest unit tests | ✅ **done** — `cargo check` + `cargo test` green |
 | **P3** | Wrap existing ADR-115 MQTT publisher as cog entry point | ✅ **wiring done** — `main.rs` boots ADR-115's `publisher::spawn` via `runtime::spawn_publisher` thin wrapper, holds a long-lived `broadcast::Sender<VitalsSnapshot>`, awaits Ctrl-C. Live-handle test green without a broker. Next (P3.5): subscribe to sensing-server `/v1/snapshot` WS and republish into the channel. |
-| **P4** | Seed-native enhancements (embedded broker, mDNS, witness) | in progress — mDNS service-record builder shipped: pure `mdns::build_mdns_service` returns wire-format `MdnsService { service_type, instance_name, control_port, txt_records }` with locked 6-key TXT surface (`cog_id`, `cog_version`, `node_id`, `mqtt_port`, `privacy`, `proto`). 9 unit tests, including PII-leak guard. Responder (mdns-sd) + embedded rumqttd + witness chain still pending. |
+| **P4** | Seed-native enhancements (embedded broker, mDNS, witness) | in progress — (a) mDNS service-record builder shipped (`mdns::build_mdns_service`, 6-key locked TXT surface, PII-leak guard). (b) Witness hash-chain primitive shipped (`witness::WitnessChain` — append-only SHA-256 chain with `verify()` catching tampered payload / broken prev_hash / seq gap). (c) Responder (mdns-sd) + embedded rumqttd + Ed25519 signing layer still pending. |
 | **P5** | RuVector-backed threshold learning (SONA adaptation) | pending |
 | **P6** | Multi-Seed federation (cross-Seed dedup + witness) | pending |
 | **P7** | Matter Bridge mode (depends on matter-rs / esp-matter readiness) | pending |