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/helpers/raw_session_retransmit.dart

@@ -9,6 +9,11 @@ typedef RawPacketSender =
       required Uint8List payload,
     });
 
+/// Pacing between consecutive raw fragments. Firmware v1.15 rejects bursts of
+/// raw packets with `ERROR: Table full` when fragments are blasted too quickly,
+/// so we space them out to keep the radio's raw transmit queue from overflowing.
+const Duration _interFragmentDelay = Duration(milliseconds: 350);
+
 Future<bool> serveCachedSessionFragments<T>({
   required String providerLabel,
   required String sessionId,
@@ -19,6 +24,7 @@ Future<bool> serveCachedSessionFragments<T>({
   required Uint8List Function(T fragment) encodeBinary,
   required RawPacketSender? sendRawPacket,
   Set<int>? requestedIndices,
+  Duration interFragmentDelay = _interFragmentDelay,
 }) async {
   if (fragments.isEmpty) {
     debugPrint('⚠️ [$providerLabel] No cached fragments for $sessionId');
@@ -56,6 +62,9 @@ Future<bool> serveCachedSessionFragments<T>({
       continue;
     }
     try {
+      if (servedCount > 0 && interFragmentDelay > Duration.zero) {
+        await Future<void>.delayed(interFragmentDelay);
+      }
       await sendRawPacket(
         contactPath: requester.outPath,
         contactPathLen: requester.routeEncodedPathLen,