feat(dht): Phase 1 — BLAKE3 content addressing alongside SHA-256

Adds the iroh-native, range-verifiable hash next to the incumbent SHA-256
so the swarm can later fetch/verify by BLAKE3 with the registry/origin as
fallback. Non-breaking: SHA-256 stays the mandatory gate; BLAKE3 is verified
only when present.

- content_hash.rs: HashAlg + ContentDigest (parse/verify '<alg>:<hex>'
  multihash strings), blake3_hex/sha256_hex; BLAKE3 known-answer test
- update.rs: ComponentUpdate.blake3 (serde-default); verified ALONGSIDE
  SHA-256 in the resumable download loop, re-download on mismatch
- blobs.rs: BlobMeta.blake3 computed on put (on-disk path stays
  SHA-256-keyed for back-compat; advertises the future swarm address)

Drive-by: fix a pre-existing stale test (test_save_and_load_state_roundtrip)
that never wrote the .download-complete marker #26 requires, so load_state's
self-heal cleared update_in_progress. Unrelated to BLAKE3 — surfaced by
running the full update:: suite.

40/40 content_hash/update/blobs tests pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

core/Cargo.lock

@@ -26,7 +26,7 @@ checksum = "b169f7a6d4742236a0a00c541b845991d0ac43e546831af1249753ab4c3aa3a0"
 dependencies = [
  "cfg-if",
  "cipher",
- "cpufeatures",
+ "cpufeatures 0.2.17",
 ]
 
 [[package]]
@@ -92,6 +92,7 @@ dependencies = [
  "bcrypt",
  "bip39",
  "bitcoin",
+ "blake3",
  "bs58",
  "bytes",
  "chacha20poly1305",
@@ -202,10 +203,16 @@ checksum = "3c3610892ee6e0cbce8ae2700349fcf8f98adb0dbfbee85aec3c9179d29cc072"
 dependencies = [
  "base64ct",
  "blake2",
- "cpufeatures",
+ "cpufeatures 0.2.17",
  "password-hash",
 ]
 
+[[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"
@@ -424,6 +431,20 @@ dependencies = [
  "digest",
 ]
 
+[[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"
@@ -524,7 +545,7 @@ checksum = "c3613f74bd2eac03dad61bd53dbe620703d4371614fe0bc3b9f04dd36fe4e818"
 dependencies = [
  "cfg-if",
  "cipher",
- "cpufeatures",
+ "cpufeatures 0.2.17",
 ]
 
 [[package]]
@@ -604,6 +625,12 @@ version = "0.3.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7c74b8349d32d297c9134b8c88677813a227df8f779daa29bfc29c183fe3dca6"
 
+[[package]]
+name = "constant_time_eq"
+version = "0.4.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3d52eff69cd5e647efe296129160853a42795992097e8af39800e1060caeea9b"
+
 [[package]]
 name = "core-foundation"
 version = "0.9.4"
@@ -629,6 +656,15 @@ 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"
@@ -686,7 +722,7 @@ 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",
@@ -1921,7 +1957,7 @@ version = "0.8.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8159bd90725d2df49889a078b54f4f79e87f1f8a8444194cdca81d38f5393abf"
 dependencies = [
- "cpufeatures",
+ "cpufeatures 0.2.17",
  "opaque-debug",
  "universal-hash",
 ]
@@ -1933,7 +1969,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9d1fe60d06143b2430aa532c94cfe9e29783047f06c0d7fd359a9a51b729fa25"
 dependencies = [
  "cfg-if",
- "cpufeatures",
+ "cpufeatures 0.2.17",
  "opaque-debug",
  "universal-hash",
 ]
@@ -2478,7 +2514,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f5058ada175748e33390e40e872bd0fe59a19f265d0158daa551c5a88a76009c"
 dependencies = [
  "cfg-if",
- "cpufeatures",
+ "cpufeatures 0.2.17",
  "digest",
 ]
 
@@ -2489,7 +2525,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e3bf829a2d51ab4a5ddf1352d8470c140cadc8301b2ae1789db023f01cedd6ba"
 dependencies = [
  "cfg-if",
- "cpufeatures",
+ "cpufeatures 0.2.17",
  "digest",
 ]
 
@@ -2506,7 +2542,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a7507d819769d01a365ab707794a4084392c824f54a7a6a7862f8c3d0892b283"
 dependencies = [
  "cfg-if",
- "cpufeatures",
+ "cpufeatures 0.2.17",
  "digest",
 ]
 
@@ -2972,7 +3008,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f124352108f58ef88299e909f6e9470f1cdc8d2a1397963901b4a6366206bf72"
 dependencies = [
  "base32",
- "constant_time_eq",
+ "constant_time_eq 0.3.1",
  "hmac",
  "rand 0.9.2",
  "sha1",