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.

test/mocks/SHA256.h

@@ -3,12 +3,33 @@
 #include <stdint.h>
 #include <stddef.h>
 
-// Mock SHA256 class for testing
-// Provides minimal interface to allow Utils.cpp to compile
+// Mock SHA256 for native testing — deterministic but not cryptographic.
+// finalize() writes real (non-garbage) output so calculatePacketHash() produces
+// distinguishable results for packets with different payloads.
+#include <string.h>
+
 class SHA256 {
+  uint8_t _state[32];
+  size_t _len;
 public:
-  void update(const uint8_t* data, size_t len) {}
-  void finalize(uint8_t* hash, size_t hashLen) {}
+  SHA256() : _len(0) { memset(_state, 0, sizeof(_state)); }
+
+  void update(const void* data, size_t len) {
+    const uint8_t* bytes = static_cast<const uint8_t*>(data);
+    for (size_t i = 0; i < len; i++) {
+      uint8_t b = bytes[i];
+      _state[_len % 32] ^= b;
+      _state[(_len + 1) % 32] += (uint8_t)((b >> 1) | (b << 7));
+      _len++;
+    }
+  }
+
+  void finalize(uint8_t* hash, size_t hashLen) {
+    for (size_t i = 0; i < hashLen; i++) {
+      hash[i] = _state[i % 32];
+    }
+  }
+
   void resetHMAC(const uint8_t* key, size_t keyLen) {}
   void finalizeHMAC(const uint8_t* key, size_t keyLen, uint8_t* hash, size_t hashLen) {}
 };