test(seed): pin known answers for the six unpinned derivations

`test_node_key_known_answer_vs_python_verifier` pinned the node Ed25519 and node
Nostr keys, and `test_release_root_known_answer` covers the release root. The
remaining six — FIPS mesh transport, identity Ed25519, identity Nostr (NIP-06),
Bitcoin BIP-84 and LND aezeed entropy — were only asserted to be mutually
distinct by `test_full_derivation_from_known_mnemonic`.

Distinctness is satisfied by ANY change to an HKDF info string or BIP-32 path.
So redefining `archipelago/lnd/entropy/v1` — the seed behind a user's Lightning
wallet — broke no test, while invalidating every backup verification a user had
already performed against docs/SEED-VERIFICATION.md. Same for the FIPS key that
authenticates a node on the mesh.

Expected values were produced independently by the Python verifier published in
that doc, whose primitives were themselves cross-checked against bip_utils and
cryptography's own HKDF (BIP-39 seed, both BIP-32 paths, x-only pubkey, bech32
and HKDF-SHA256 salt=None all matched byte for byte). This commit closes the
loop in the other direction: the Rust implementation now agrees with those same
bytes, so the doc and the code are pinned to each other.

Verified: 26/26 seed tests pass.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

core/archipelago/src/seed.rs

@@ -919,6 +919,79 @@ mod tests {
         );
     }
 
+    /// Known answers for the derivations the test above does NOT pin.
+    ///
+    /// `test_full_derivation_from_known_mnemonic` only asserts these are
+    /// mutually distinct, which is satisfied by ANY change to an HKDF info
+    /// string or BIP-32 path. That left the LND entropy — the seed behind a
+    /// user's Lightning wallet — and the FIPS mesh transport key with no
+    /// known-answer coverage at all: silently redefining either broke no test
+    /// while invalidating every backup verification a user had already done.
+    ///
+    /// The expected values were produced independently by the Python verifier
+    /// published in `docs/SEED-VERIFICATION.md`, whose primitives were in turn
+    /// cross-checked against `bip_utils` and `cryptography`'s own HKDF. If one
+    /// of these assertions fails, either a derivation changed (and every
+    /// published backup-verification instruction is now wrong), or the doc and
+    /// the code have drifted apart — both are release-blocking.
+    #[test]
+    fn test_all_derivations_known_answers_vs_python_verifier() {
+        let (_, seed) = MasterSeed::from_mnemonic_words(TEST_MNEMONIC).unwrap();
+
+        // HKDF "archipelago/fips/secp256k1/v1"
+        assert_eq!(
+            derive_fips_key(&seed).unwrap().public_key().to_hex(),
+            "31865360f8cfb8bc3d0d5343bf09f1bc5c17b7f0b509120e94cdbed47d203bca",
+            "FIPS mesh transport key"
+        );
+
+        // HKDF "archipelago/identity/{i}/ed25519/v1"
+        assert_eq!(
+            hex::encode(
+                derive_identity_ed25519(&seed, 0)
+                    .unwrap()
+                    .verifying_key()
+                    .as_bytes()
+            ),
+            "c0415866117dd570e908e1b178bf1dd8f7b9cdb8eb662353165c6f4877fe222b",
+            "identity[0] Ed25519"
+        );
+        assert_eq!(
+            hex::encode(
+                derive_identity_ed25519(&seed, 1)
+                    .unwrap()
+                    .verifying_key()
+                    .as_bytes()
+            ),
+            "be7a0e1fe2711dedbbba976a70443e0a70e673bcf7b6e6a8fb20d1d2684e5eba",
+            "identity[1] Ed25519 — pins the index into the info string"
+        );
+
+        // BIP-32 m/44'/1237'/0'/0/{i} (NIP-06)
+        assert_eq!(
+            derive_nostr_identity_key(&seed, 0)
+                .unwrap()
+                .public_key()
+                .to_hex(),
+            "1ca7e48a33d62063f25d79f18617cc892ef064628aaf80af485e41849343ca52",
+            "identity[0] Nostr (NIP-06)"
+        );
+
+        // BIP-32 m/84'/0'/0'
+        assert_eq!(
+            hex::encode(derive_bitcoin_xprv(&seed).unwrap().private_key.secret_bytes()),
+            "57558e8c90c2e72f0c121d0fb8844bbbe7a872f0065d21b218a990450b9f93be",
+            "Bitcoin BIP-84 account key"
+        );
+
+        // HKDF "archipelago/lnd/entropy/v1" (16 bytes)
+        assert_eq!(
+            hex::encode(derive_lnd_entropy(&seed).unwrap()),
+            "5c86f10629bd86cdd269b82a76cc51e4",
+            "LND aezeed entropy"
+        );
+    }
+
     #[test]
     fn test_release_root_deterministic_and_domain_separated() {
         let (_, seed) = MasterSeed::from_mnemonic_words(TEST_MNEMONIC).unwrap();