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/fips/mod.rs

@@ -180,7 +180,10 @@ mod tests {
         // anchor is the only candidate.
         let status = FipsStatus::query(dir.path()).await;
         assert!(!status.key_present, "no key before onboarding");
-        assert!(status.npub.is_none());
+        // `npub` falls back to whatever an already-running local fips
+        // daemon advertises, so on a dev machine or node with fips
+        // installed this field can be Some(...) even when the test
+        // data_dir is empty. We only assert that key_present is false.
         // `installed`, `service_state`, `version` depend on the host and are
         // not asserted here — query() must return cleanly regardless.
     }