refactor: split MeshTables::hasSeen into pure query + markSeen

hasSeen() was simultaneously a predicate and a mutator — it inserted the
packet hash on every miss, making five call sites that only wanted to mark
a packet as sent call it with the return value discarded.

Split into:
- wasSeen()   — pure predicate, no side effects
- markSeen()  — explicit insert

All query sites now call markSeen() immediately after wasSeen() returns
false, preserving identical runtime behaviour. The five mark-only send
sites (sendFlood, sendDirect, sendZeroHop x2) now call markSeen directly.

Also fixes three bridge sites (BridgeBase, ESPNowBridge, RS232Bridge)
that had the same query+implicit-insert pattern.

Tests: add test/test_mesh_tables/ covering wasSeen purity, markSeen,
dup stats, and clear. Update SHA256 mock to produce deterministic output
(previously finalize() was a no-op). Add Packet.cpp to native build filter.

src/helpers/bridges/ESPNowBridge.cpp

@@ -167,7 +167,8 @@ void ESPNowBridge::sendPacket(mesh::Packet *packet) {
     return;
   }
 
-  if (!_seen_packets.hasSeen(packet)) {
+  if (!_seen_packets.wasSeen(packet)) {
+    _seen_packets.markSeen(packet);
     // Create a temporary buffer just for size calculation and reuse for actual writing
     uint8_t sizingBuffer[MAX_PAYLOAD_SIZE];
     uint16_t meshPacketLen = packet->writeTo(sizingBuffer);