docs(SEED-VERIFICATION): add the missing FIPS key, fix the comparison commands

Ran the doc's script rather than only reading it, and cross-checked every
primitive it implements against independent libraries (bip_utils for BIP-39
seed / BIP-32 derivation / bech32, and cryptography's own HKDF): BIP-39 seed,
m/44'/1237'/0'/0/0, m/84'/0'/0', x-only pubkey, npub encoding and
HKDF-SHA256(salt=None) all match byte for byte. The hand-rolled crypto in this
doc is correct.

Two real gaps fixed:

- **The FIPS mesh transport key was missing.** `seed.rs:227` derives it from the
  same master seed via `archipelago/fips/secp256k1/v1`, and a user verifying
  their backup had no way to check it — despite it being the key that
  authenticates them on the mesh. Added it to the diagram and as section 2b of
  the script (same shape as the node Nostr key; verified against
  `derive_fips_key` and `hkdf_derive` using `Hkdf::new(None, ikm)`).
- **The "compare with your node" commands were wrong.** The RPC endpoint is
  `/rpc/v1`, not `/api/rpc`, and `identity.get-node` is not a method — the real
  ones are `node.did` and `node.nostr-pubkey`. Also dropped "UI: Settings >
  Identity", which is not a screen that exists, in favour of the two
  identity files on disk.

Verified and left alone: all five other HKDF info strings, both BIP-32 paths,
and the `node_key.pub` filename. The release-root key
(`archipelago/release/root/ed25519/v1`) is deliberately still absent — it is
derived from the project's signing seed, not a user's node seed.

Noted separately: `system.get-node-key` sits in the CSRF-exempt list
(`api/rpc/mod.rs:337`) but has no dispatcher arm, so it is an exemption for a
method that does not exist. Harmless, but it should be removed.

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

docs/SEED-VERIFICATION.md

@@ -19,6 +19,9 @@ PBKDF2-HMAC-SHA512 (2048 rounds, salt = "mnemonic")
       +-- HKDF-SHA256 (info="archipelago/nostr-node/secp256k1/v1")
       |       --> Node Nostr key --> npub1...
       |
+      +-- HKDF-SHA256 (info="archipelago/fips/secp256k1/v1")
+      |       --> FIPS mesh transport key --> npub1...
+      |
       +-- HKDF-SHA256 (info="archipelago/identity/{i}/ed25519/v1")
       |       --> Identity[i] Ed25519 --> did:key:z...
       |
@@ -265,6 +268,20 @@ def main():
     print(f"  nsec:     {bech32_encode('nsec', node_nostr_priv)}")
     print(f"  npub:     {bech32_encode('npub', node_nostr_pub)}")
 
+    # ── 2b. FIPS mesh transport key ─────────────────────────────────────
+
+    print()
+    print("-" * W)
+    print("  2b. FIPS MESH TRANSPORT KEY")
+    print(f"     HKDF-SHA256(seed, info='archipelago/fips/secp256k1/v1')")
+    print("-" * W)
+
+    fips_priv = hkdf_sha256(seed, b"archipelago/fips/secp256k1/v1")
+    fips_pub = secp256k1_xonly(fips_priv)
+
+    print(f"  X-only:   {fips_pub.hex()}")
+    print(f"  npub:     {bech32_encode('npub', fips_pub)}")
+
     # ── 3. Identity[0..2] Ed25519 + DID ─────────────────────────────────
 
     print()
@@ -324,10 +341,12 @@ def main():
     print()
     print("=" * W)
     print("  Compare these values with your Archipelago node:")
-    print("    UI:  Settings > Identity")
-    print("    SSH: xxd -p /var/lib/archipelago/identity/node_key.pub")
-    print("    RPC: curl -s http://<ip>/api/rpc \\")
-    print("           -d '{\"method\":\"identity.get-node\"}' | jq .")
+    print("    SSH: xxd -p /var/lib/archipelago/identity/node_key.pub   (section 1)")
+    print("         cat /var/lib/archipelago/identity/nostr_pubkey      (section 2)")
+    print("    RPC: curl -s -b jar.txt http://<ip>/rpc/v1 \\")
+    print("           -H 'Content-Type: application/json' \\")
+    print("           -d '{\"method\":\"node.did\"}' | jq .")
+    print("         ...and {\"method\":\"node.nostr-pubkey\"} for the npub")
     print("=" * W)
     print()