fix: Normalize raw transport path for firmware v1.15 photo/audio transfer

CMD_SEND_RAW_DATA (0x19) was passed the encoded MeshCore route descriptor
(e.g. 0x41 for a 1-hop 2-byte-hash route), but the v1.15 companion firmware
raw-data handler treats the path byte as a legacy literal hop count, not an
encoded descriptor. It misread 0x41 as "65 path bytes" and rejected the
command (ERROR: Unsupported command), so media fetch always failed while
normal messaging worked.

- Add ContactRouteCodec.toLegacyRawPath() converting encoded descriptor +
  path into legacy format (literal hop count + first byte of each hop hash).
- Apply it in ConnectionProvider.sendRawVoicePacket, the single chokepoint
  all raw media (voice/image fragments, swarm, route probes) flows through.
- Pace served fragments 350ms apart to avoid firmware "ERROR: Table full"
  when bursting many raw packets.
- Tests for the descriptor conversion.

Fixes #43

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/providers/connection_provider.dart

@@ -1833,15 +1833,23 @@ class ConnectionProvider with ChangeNotifier {
 
   /// Send a raw binary voice packet directly to a contact (cmdSendRawData, code 25).
   /// Only works for contacts with a known direct route (outPathLen >= 0).
+  ///
+  /// Callers pass the contact's *encoded* route descriptor ([routeEncodedPathLen])
+  /// and encoded [outPath]. The `CMD_SEND_RAW_DATA` handler in companion firmware
+  /// v1.15 misinterprets encoded descriptors as a literal path-byte count, so we
+  /// normalise to the legacy raw path format (literal hop count + one byte per
+  /// hop) before handing the frame to the transport. See
+  /// [ContactRouteCodec.toLegacyRawPath].
   Future<void> sendRawVoicePacket({
     required Uint8List contactPath,
     required int contactPathLen,
     required Uint8List payload,
   }) async {
     if (!_activeService.isConnected) return;
+    final legacy = ContactRouteCodec.toLegacyRawPath(contactPathLen, contactPath);
     await _activeService.sendRawVoicePacket(
-      contactPathLen: contactPathLen,
-      contactPath: contactPath,
+      contactPathLen: legacy.length,
+      contactPath: Uint8List.fromList(legacy.path),
       payload: payload,
     );
   }