feat: replace advert scan with active node discovery in NearbyScreen

NearbyScreen now sends CTL_TYPE_NODE_DISCOVER_REQ (same protocol as the
companion app) instead of a plain self-advert. Nearby repeaters, sensors
and room servers respond with their pub_key and type. Known contacts get
their lastmod refreshed; unknown nodes appear as temporary entries (e.g.
"Repeater") in the list until their advert is received.

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

examples/companion_radio/MyMesh.h

@@ -85,6 +85,12 @@ struct AdvertPath {
   uint8_t path[MAX_PATH_SIZE];
 };
 
+struct DiscoveredEntry {
+  uint8_t pub_key_prefix[4];
+  uint8_t type;       // ADV_TYPE_REPEATER / ADV_TYPE_SENSOR / ADV_TYPE_ROOM
+  uint32_t timestamp; // RTC timestamp of discovery
+};
+
 #define EXPECTED_ACK_TABLE_SIZE 8
 
 class MyMesh : public BaseChatMesh, public DataStoreHost {
@@ -101,9 +107,11 @@ public:
   void loop();
   void handleCmdFrame(size_t len);
   bool advert();
+  void sendNodeDiscoverReq();
   void enterCLIRescue();
 
   int  getRecentlyHeard(AdvertPath dest[], int max_num);
+  int  getDiscoveredNodes(DiscoveredEntry dest[], int max_count);
 
 protected:
   float getAirtimeBudgetFactor() const override;
@@ -263,6 +271,12 @@ private:
 
   #define ADVERT_PATH_TABLE_SIZE   16
   AdvertPath advert_paths[ADVERT_PATH_TABLE_SIZE]; // circular table
+
+  #define DISCOVERED_NODES_MAX 8
+  DiscoveredEntry _discovered[DISCOVERED_NODES_MAX];
+  int             _discovered_count;
+  uint32_t        _pending_node_discover_tag;
+  unsigned long   _pending_node_discover_until;
 };
 
 extern MyMesh the_mesh;