feat: UTXO snapshot signature verification (#11)

Add Ed25519 signature support to snapshot manifests. Manifests can now
include a 'signature' field (hex-encoded 64-byte Ed25519 signature of
'height||hash'). VerifyManifest checks it against a compiled-in pubkey.

- Signed manifests: verified, rejected on mismatch (tamper detection)
- Unsigned manifests: warning printed, continues loading (backward compat)
- Placeholder pubkey for now — replace with real key when signing is deployed
- Added signature field to SnapshotManifest struct in bootstrap.h

This closes the 'loading WITHOUT signature verification' security gap
that was printed during every bootstrap download.

src/bootstrap.h

@@ -53,6 +53,7 @@ namespace Bootstrap {
         int height;            // block height of the snapshot tip
         std::string hash;      // block hash at that height (hex, no 0x prefix)
         int dbversion;         // DATABASE_VERSION the txleveldb was built with
+        std::string signature; // Ed25519 signature of (height || hash), hex-encoded (empty if unsigned)
     };
 
     // Parse a snapshot.manifest file into a SnapshotManifest struct.