feat(#176): OFFBAND_CAP_BLOCK detection (supportsOffbandBlock)

offbandCapBlock=0x02 + firmwareSupportsOffbandBlock(caps, verCode): requires the
explicit bit AND FIRMWARE_VER_CODE>=15 (per firmware as-built PR #247). Connector
exposes supportsOffbandBlock; absent -> app-local block only. Mirrors the 0xC1
GPS capability pattern.

Epic B #166 / B1 #176. Design: docs/architecture/block-contract-as-built.md.

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

lib/connector/meshcore_connector.dart

@@ -540,6 +540,11 @@ class MeshCoreConnector extends ChangeNotifier {
   /// advertised the Offband-fork `offband_caps` byte. Stock MeshCore omits it,
   /// so 0xC1 is suppressed there rather than pinged blindly. (#144)
   bool get supportsOffbandGps => firmwareSupportsOffbandGps(_offbandCaps);
+
+  /// Whether the connected radio persists a block list + drops blocked DMs
+  /// (Offband firmware, capability-gated). Absent → app-local block only.
+  bool get supportsOffbandBlock =>
+      firmwareSupportsOffbandBlock(_offbandCaps, _firmwareVerCode);
   Map<String, String>? get currentCustomVars => _currentCustomVars;
   int? get batteryMillivolts => _batteryMillivolts;
   int? get storageUsedKb => _storageUsedKb;

lib/connector/meshcore_protocol.dart

@@ -274,6 +274,18 @@ Uint8List buildOffbandGpsRequestFrame() => Uint8List.fromList([cmdOffbandGps]);
 /// defines one, any Offband v14+ radio is assumed to speak `0xC1`.
 bool firmwareSupportsOffbandGps(int? offbandCaps) => offbandCaps != null;
 
+/// `OFFBAND_CAP_BLOCK` bit (bit 1) in the `offband_caps` byte of the device-info
+/// reply: an Offband radio that persists a block list and drops blocked DMs at
+/// receive (firmware PR #247, `FIRMWARE_VER_CODE 15`). Unlike the loose GPS
+/// presence-gate above, block requires the **explicit bit** set AND
+/// `FIRMWARE_VER_CODE >= 15`; absent → app-only mode (no sync, no firmware drop).
+const int offbandCapBlock = 0x02;
+
+bool firmwareSupportsOffbandBlock(int? offbandCaps, int? firmwareVerCode) =>
+    offbandCaps != null &&
+    (offbandCaps & offbandCapBlock) != 0 &&
+    (firmwareVerCode ?? 0) >= 15;
+
 const int statsTypeCore = 0;
 const int statsTypeRadio = 1;
 const int statsTypePackets = 2;