cog-ha-matter (ADR-116 P4): Ed25519 signing layer for witness chain

Closes the cryptographic-attestation gap in ADR-116 §2.2: every
witness event can now be signed by the Seed's Ed25519 key, with
verify available to any auditor holding the public key.

Module shape (`src/witness_signing.rs`, kept separate from
`witness::` so the hash chain stays usable without dalek linked
in — important for the wasm32 audit-verifier variant we'll ship
later):

  * sign_event(event, &SigningKey) -> Signature
  * verify_signature(event, &Signature, &VerifyingKey)
        -> Result<(), SignatureVerifyError>
  * signature_to_hex / signature_from_hex (128-char lowercase,
    matches the witness hex convention)
  * SignatureVerifyError::Invalid
  * SignatureParseError::{Length, Hex}

Key design point: signature covers the SAME canonical bytes
witness::hash_event hashes. That means:

  1. A signed event commits to the entire event content (kind,
     payload, timestamp, seq, prev_hash) — no field can be
     retroactively changed without invalidating both the hash AND
     the signature.

  2. The signature implicitly commits to the event's *chain
     position* via prev_hash — splicing a signed event into a
     different chain breaks verification.

Adds `ed25519-dalek = "2.1"` to cog-ha-matter (already in
workspace via ruv-neural, version kept aligned).

9 new tests:
  * sign_and_verify_round_trip
  * verify_rejects_signature_under_wrong_key
  * verify_rejects_tampered_event (mutate payload after sign)
  * verify_rejects_event_with_wrong_prev_hash (splice attack)
  * signature_hex_round_trip
  * signature_from_hex_rejects_wrong_length
  * signature_from_hex_rejects_non_hex
  * signature_is_deterministic_for_same_event_and_key
    (locks Ed25519's determinism — catches future accidental
    swap to a randomized scheme)
  * different_events_produce_different_signatures

60/60 cog tests green (51 → 60). Key management is intentionally
out of scope here — the cog runtime reads the Seed's key from the
Cognitum control plane's secure store (separate concern).

ADR-116 P4 now ⁵⁄₆: ✅ mDNS record, ✅ chain, ✅ JSONL, ✅ file
persistence, ✅ Ed25519 signing; ⏳ responder + embedded broker.

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

v2/Cargo.lock

@@ -934,6 +934,7 @@ name = "cog-ha-matter"
 version = "0.3.0"
 dependencies = [
  "clap",
+ "ed25519-dalek",
  "serde",
  "serde_json",
  "sha2",
@@ -1074,6 +1075,12 @@ dependencies = [
  "wasm-bindgen",
 ]
 
+[[package]]
+name = "const-oid"
+version = "0.9.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c2459377285ad874054d797f3ccebf984978aa39129f6eafde5cdc8315b612f8"
+
 [[package]]
 name = "constant_time_eq"
 version = "0.1.5"
@@ -1367,6 +1374,33 @@ dependencies = [
  "libloading 0.9.0",
 ]
 
+[[package]]
+name = "curve25519-dalek"
+version = "4.1.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "97fb8b7c4503de7d6ae7b42ab72a5a59857b4c937ec27a3d4539dba95b5ab2be"
+dependencies = [
+ "cfg-if",
+ "cpufeatures",
+ "curve25519-dalek-derive",
+ "digest",
+ "fiat-crypto",
+ "rustc_version",
+ "subtle",
+ "zeroize",
+]
+
+[[package]]
+name = "curve25519-dalek-derive"
+version = "0.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f46882e17999c6cc590af592290432be3bce0428cb0d5f8b6715e4dc7b383eb3"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.117",
+]
+
 [[package]]
 name = "darling"
 version = "0.21.3"
@@ -1428,6 +1462,7 @@ version = "0.7.10"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e7c1832837b905bbfb5101e07cc24c8deddf52f93225eee6ead5f4d63d53ddcb"
 dependencies = [
+ "const-oid",
  "pem-rfc7468",
  "zeroize",
 ]
@@ -1643,6 +1678,30 @@ dependencies = [
  "num-traits",
 ]
 
+[[package]]
+name = "ed25519"
+version = "2.2.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "115531babc129696a58c64a4fef0a8bf9e9698629fb97e9e40767d235cfbcd53"
+dependencies = [
+ "pkcs8",
+ "signature",
+]
+
+[[package]]
+name = "ed25519-dalek"
+version = "2.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "70e796c081cee67dc755e1a36a0a172b897fab85fc3f6bc48307991f64e4eca9"
+dependencies = [
+ "curve25519-dalek",
+ "ed25519",
+ "serde",
+ "sha2",
+ "subtle",
+ "zeroize",
+]
+
 [[package]]
 name = "either"
 version = "1.15.0"
@@ -1773,6 +1832,12 @@ dependencies = [
  "simd-adler32",
 ]
 
+[[package]]
+name = "fiat-crypto"
+version = "0.2.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "28dea519a9695b9977216879a3ebfddf92f1c08c05d984f8996aecd6ecdc811d"
+
 [[package]]
 name = "field-offset"
 version = "0.3.6"
@@ -5097,6 +5162,16 @@ version = "0.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8b870d8c151b6f2fb93e84a13146138f05d02ed11c7e7c54f8826aaaf7c9f184"
 
+[[package]]
+name = "pkcs8"
+version = "0.10.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f950b2377845cebe5cf8b5165cb3cc1a5e0fa5cfa3e1f7f55707d8fd82e0a7b7"
+dependencies = [
+ "der",
+ "spki",
+]
+
 [[package]]
 name = "pkg-config"
 version = "0.3.32"
@@ -6996,6 +7071,15 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "signature"
+version = "2.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "77549399552de45a898a580c1b41d445bf730df867cc44e6c0233bbc4b8329de"
+dependencies = [
+ "rand_core 0.6.4",
+]
+
 [[package]]
 name = "simba"
 version = "0.9.1"
@@ -7154,6 +7238,16 @@ dependencies = [
  "lock_api",
 ]
 
+[[package]]
+name = "spki"
+version = "0.7.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d91ed6c858b01f942cd56b37a94b3e0a1798290327d1236e4d9cf4eaca44d29d"
+dependencies = [
+ "base64ct",
+ "der",
+]
+
 [[package]]
 name = "stable_deref_trait"
 version = "1.2.1"