feat: show RSSI in discover results list

Add rssi field to DiscoverResult, captured from _radio->getLastRSSI()
when the discover response is received.

Display format in the right column:
  known node  → "Rpt-87" / "Snsr-92" (type + dBm)
  new node    → "*-87" (asterisk + dBm; type already visible in "[Rpt]" label)

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

examples/companion_radio/MyMesh.h

@@ -89,6 +89,7 @@ struct DiscoverResult {
   char    name[32];   // contact name if known, "" if unknown (use type label)
   uint8_t type;       // ADV_TYPE_REPEATER / ADV_TYPE_SENSOR / ADV_TYPE_ROOM
   bool    is_known;   // true = in contacts[], false = new unknown node
+  int8_t  rssi;       // RSSI of the response as received by us (dBm)
   uint32_t timestamp;
 };