feat(firmware): runtime PHY lock + drop post-TX self-RX on announces

- "LOCK:<T|C|R>" serial command pins the radio to one PHY for debugging /
  focused RX testing; "LOCK:" resumes the normal RTC scan
- clear rxFlag after a Reticulum announce TX so the TX-done interrupt no longer
  makes the bridge log a phantom "reception" of its own packet

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

firmware/src/main.cpp

@@ -105,6 +105,7 @@ static int      activeIdx = 0;
 static uint32_t totalPkts = 0;
 static uint32_t dwellStart = 0;
 static uint32_t frame = 0;
+static char     lockNet = 0;   // if T/C/R, pin the radio to that PHY (debug/test)
 
 // Outbound message queued from the serial console ("T:hello", or bare "hello"
 // == all nets). Flushed on the matching PHY window so the radio is tuned right.
@@ -512,6 +513,7 @@ static void sendReticulumAnnounce() {
   for (size_t i = 0; i < p; i++) Serial.printf("%02x", pkt[i]);   // for RNS cross-validation
   Serial.println();
   radio.startReceive();
+  rxFlag = false;   // discard the TX-done interrupt so we don't self-RX our packet
 }
 
 // Known Reticulum destinations we've heard announces from.
@@ -1022,6 +1024,17 @@ static void pollSerialTx() {
         n = 0;
         continue;
       }
+      // "LOCK:R" pins the radio to one PHY for testing; "LOCK:" resumes scanning.
+      if (n >= 5 && strncmp(buf, "LOCK:", 5) == 0) {
+        char L = toupper(buf[5]);
+        if (L == 'T' || L == 'C' || L == 'R') {
+          for (size_t k = 0; k < NUM_CONFIGS; k++)
+            if (CONFIGS[k].letter == L) { activeIdx = (int)k; applyConfig(CONFIGS[k]); }
+          lockNet = L; dwellStart = millis();
+          Serial.printf("  [lock] pinned to %c\n", L);
+        } else { lockNet = 0; Serial.println("  [lock] released — scanning"); }
+        n = 0; continue;
+      }
       if (n > 0) {
         const char* msg = buf;
         strcpy(txNets, "TCR");            // default: all networks
@@ -1069,8 +1082,8 @@ void loop() {
   pollSerialTx();
 
   // Non-blocking scan: rotate the radio when the dwell window elapses, so the
-  // display keeps animating smoothly the whole time.
+  // display keeps animating smoothly the whole time. Skipped while pinned.
-  if (millis() - dwellStart >= DWELL_MS) {
+  if (!lockNet && millis() - dwellStart >= DWELL_MS) {
     activeIdx = (activeIdx + 1) % NUM_CONFIGS;
     applyConfig(CONFIGS[activeIdx]);
     dwellStart = millis();