feat(adr-118/p3.5): SignatureHasher (BLAKE3-keyed) — 117/117 GREEN

Iter 15. Lands ADR-120 §2.3 — the cryptographic foundation of invariant
I3 ("cross-site identity correlation is impossible"). rf_signature_hash
is now derived from a per-site secret and a daily epoch, so two nodes
observing the same physical person produce uncorrelated 256-bit digests.

Added (no_std-compatible):
- blake3 = "1.5", default-features = false (no_std, no SIMD by default)
- src/signature_hasher.rs:
  * Constants SECONDS_PER_DAY (86_400), SITE_SALT_LEN (32), RF_SIGNATURE_LEN (32)
  * SignatureHasher { site_salt: [u8; 32] } with new(salt) const ctor
  * compute(day_epoch, &features) -> [u8; 32]  (BLAKE3 keyed mode)
  * compute_at(unix_secs, &features) -> [u8; 32] convenience
  * day_epoch_from_unix_secs(unix_secs) -> u32 helper (floor(t / 86400))
- pub use SignatureHasher, RF_SIGNATURE_LEN, SITE_SALT_LEN from lib.rs

tests/signature_hasher.rs (8 named tests, all green):
  deterministic_under_identical_inputs
  different_site_salts_produce_different_hashes
  different_day_epochs_rotate_the_hash
  different_features_produce_different_hashes
  output_length_is_32_bytes
  day_epoch_from_unix_secs_matches_floor_division
    (covers 0, 86_399, 86_400, and the 1.7e9 modern timestamp)
  compute_at_matches_compute_with_derived_day
  cross_site_hamming_distance_is_statistically_high
    *** ADR-120 §2.7 AC2 acceptance test ***
    Runs 100 trials with distinct (salt_a, salt_b) pairs observing
    identical features, computes per-trial Hamming distance, asserts
    mean >= 120 bits and min >= 80 bits. Empirically lands at ~128 bits
    mean (the expected value for two independent 256-bit hashes), with
    no trial below 80 bits — i.e., zero suspicious near-collisions.

ACs progressed:
- ADR-120 §2.7 AC2 — structurally enforced cross-site isolation, now
  proven empirically by the Hamming-distance test. This is the
  cryptographic half of invariant I3 in code, not just docs.
- ADR-118 invariant I3 — first runtime witness that two sites with
  independent site_salts cannot correlate the same person's signature.

Test config:
- cargo test --no-default-features → 72 passed (64 + 8; signature_hasher is no_std)
- cargo test                       → 117 passed (109 + 8)

Out of scope (next iter target):
- Wire SignatureHasher into BfldEmitter: replace caller-supplied
  rf_signature_hash with hasher.compute_at(ts, &features) so the
  pipeline produces correct hashes end-to-end.
- IdentityFeatures canonical-bytes encoder so callers don't need to
  hand-serialize per-feature representations.

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

v2/Cargo.lock

@@ -16,7 +16,7 @@ checksum = "b169f7a6d4742236a0a00c541b845991d0ac43e546831af1249753ab4c3aa3a0"
 dependencies = [
  "cfg-if",
  "cipher",
- "cpufeatures",
+ "cpufeatures 0.2.17",
 ]
 
 [[package]]
@@ -198,6 +198,12 @@ dependencies = [
  "derive_arbitrary",
 ]
 
+[[package]]
+name = "arrayref"
+version = "0.3.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "76a2e8124351fda1ef8aaaa3bbd7ebbcb486bbcd4225aca0aa0d84bb2db8fecb"
+
 [[package]]
 name = "arrayvec"
 version = "0.7.6"
@@ -456,6 +462,20 @@ dependencies = [
  "serde_core",
 ]
 
+[[package]]
+name = "blake3"
+version = "1.8.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0aa83c34e62843d924f905e0f5c866eb1dd6545fc4d719e803d9ba6030371fce"
+dependencies = [
+ "arrayref",
+ "arrayvec",
+ "cc",
+ "cfg-if",
+ "constant_time_eq 0.4.2",
+ "cpufeatures 0.3.0",
+]
+
 [[package]]
 name = "block-buffer"
 version = "0.10.4"
@@ -1088,6 +1108,12 @@ version = "0.1.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "245097e9a4535ee1e3e3931fcfcd55a796a44c643e8596ff6566d68f09b87bbc"
 
+[[package]]
+name = "constant_time_eq"
+version = "0.4.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3d52eff69cd5e647efe296129160853a42795992097e8af39800e1060caeea9b"
+
 [[package]]
 name = "convert_case"
 version = "0.4.0"
@@ -1173,6 +1199,15 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "cpufeatures"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8b2a41393f66f16b0823bb79094d54ac5fbd34ab292ddafb9a0456ac9f87d201"
+dependencies = [
+ "libc",
+]
+
 [[package]]
 name = "crc"
 version = "3.4.0"
@@ -1397,7 +1432,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "97fb8b7c4503de7d6ae7b42ab72a5a59857b4c937ec27a3d4539dba95b5ab2be"
 dependencies = [
  "cfg-if",
- "cpufeatures",
+ "cpufeatures 0.2.17",
  "curve25519-dalek-derive",
  "digest",
  "fiat-crypto",
@@ -7015,7 +7050,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e3bf829a2d51ab4a5ddf1352d8470c140cadc8301b2ae1789db023f01cedd6ba"
 dependencies = [
  "cfg-if",
- "cpufeatures",
+ "cpufeatures 0.2.17",
  "digest",
 ]
 
@@ -7026,7 +7061,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a7507d819769d01a365ab707794a4084392c824f54a7a6a7862f8c3d0892b283"
 dependencies = [
  "cfg-if",
- "cpufeatures",
+ "cpufeatures 0.2.17",
  "digest",
 ]
 
@@ -9158,6 +9193,7 @@ checksum = "72069c3113ab32ab29e5584db3c6ec55d416895e60715417b5b883a357c3e471"
 name = "wifi-densepose-bfld"
 version = "0.3.0"
 dependencies = [
+ "blake3",
  "crc",
  "proptest",
  "serde",
@@ -10412,7 +10448,7 @@ dependencies = [
  "aes",
  "byteorder",
  "bzip2",
- "constant_time_eq",
+ "constant_time_eq 0.1.5",
  "crc32fast",
  "crossbeam-utils",
  "flate2",