fix: wire NearbyScreen ping through expected_ack_table so RTT is measurable

Direct sendMessage() calls bypass the BT serial handler that populates
expected_ack_table; add registerExpectedAck() to MyMesh and call it from
NearbyScreen so isAckPending() can actually track the outstanding ping.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

examples/companion_radio/MyMesh.h

@@ -176,6 +176,14 @@ public:
     return false;
   }
 
+  void registerExpectedAck(uint32_t expected_ack) {
+    if (!expected_ack) return;
+    expected_ack_table[next_ack_idx].msg_sent = millis();
+    expected_ack_table[next_ack_idx].ack      = expected_ack;
+    expected_ack_table[next_ack_idx].contact  = nullptr;
+    next_ack_idx = (next_ack_idx + 1) % EXPECTED_ACK_TABLE_SIZE;
+  }
+
 #if ENV_INCLUDE_GPS == 1
   void applyGpsPrefs() {
     sensors.setSettingValue("gps", _prefs.gps_enabled ? "1" : "0");

examples/companion_radio/ui-new/NearbyScreen.h

@@ -298,6 +298,7 @@ public:
           uint32_t expected_ack, est_timeout;
           int res = the_mesh.sendMessage(ci, rtc_clock.getCurrentTime(), 0, "", expected_ack, est_timeout);
           if (res != MSG_SEND_FAILED && expected_ack != 0) {
+            the_mesh.registerExpectedAck(expected_ack);
             _ping_expected_ack = expected_ack;
             _ping_start_ms     = millis();
             _ping_ack_seen     = false;