feat(identity): seed-derivation verifier + KAT; rename "Your DID"→"Node DID"

- scripts/verify-seed-derivation.py: stdlib-only tool to cryptographically prove
  a node's on-disk keys (node_key→DID, nostr_secret→npub, fips_key) are derived
  from its onboarding seed exactly as seed.rs documents (BIP-39 → PBKDF2-HMAC-
  SHA512 → HKDF-SHA256 with per-key domain separation).
- seed.rs: known-answer regression test cross-checking Rust node_key + nostr
  bytes against the Python verifier (locks the derivation).
- en.json: "Your DID" → "Node DID".

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

neode-ui/src/locales/en.json

@@ -179,7 +179,7 @@
     "aiDataAccess": "AI Data Access",
     "serverName": "Hostname",
     "sessionStatus": "Session Status",
-    "yourDid": "Your DID",
+    "yourDid": "Node DID",
     "onionAddress": "Node .onion Address",
     "loggedIn": "Currently logged in",
     "didHelper": "Decentralized identifier for passwordless auth",