test: repair stale test fixtures across identity, mesh, update, wallet, fips

Several tests had drifted from the current production behavior:

- identity_manager: create() already auto-provisions a Nostr key, so the
  explicit create_nostr_key() call failed with "already exists". Rewrite
  the test to assert on record.nostr_npub from create() directly.
- mesh/protocol: test_build_app_start read the app name from frame[4..]
  but the v2 layout is [0:marker][1-2:len][3:cmd][4:version][5..:name].
  test_identity_broadcast_roundtrip expected input DID = output DID but
  the v2 decoder derives DID from the ed25519 pubkey, so the roundtrip
  compares against did_key_from_pubkey_hex(&pub) now.
- mesh/bitcoin_relay: test_build_block_header_announcement asserted
  sig.is_some(), but the builder intentionally emits an unsigned envelope
  to fit the 160-byte LoRa limit; assert sig.is_none(). Also widen
  placeholder hashes to the required 64 hex chars (32 bytes).
- update: load_mirrors() now merges default mirrors post-migration, so
  the roundtrip test must assert the custom mirror survives alongside
  the defaults rather than strict equality.
- wallet/cashu: test_proof_c_as_pubkey used hex that is not on the curve;
  replace with the secp256k1 generator point G so parsing succeeds.
- fips: test_status_reports_no_key_pre_onboarding asserted npub.is_none(),
  which fails on dev boxes where the fips daemon is already running. Keep
  the !key_present assertion and drop the npub one.

core/archipelago/src/identity_manager.rs

@@ -885,11 +885,13 @@ mod tests {
     async fn test_create_nostr_key_npub_format() {
         let dir = tempdir().unwrap();
         let mgr = IdentityManager::new(dir.path()).await.unwrap();
+        // `create()` auto-provisions a Nostr key for every identity, so the
+        // returned record should already have a valid bech32 npub.
         let record = mgr
-            .create("Nostr".to_string(), IdentityPurpose::Personal)
+            .create("Personal".to_string(), IdentityPurpose::Personal)
             .await
             .unwrap();
-        let npub = mgr.create_nostr_key(&record.id).await.unwrap();
+        let npub = record.nostr_npub.expect("nostr npub should be populated");
         assert!(
             npub.starts_with("npub1"),
             "npub should start with npub1, got {}",