feat: add dedicated discover sub-screen to NearbyScreen

Replaces the inline advert scan in the nearby list with a proper
CTL_TYPE_NODE_DISCOVER_RESP-based sub-screen accessible via the context menu.

- DiscoveredEntry → DiscoverResult: adds name[32] and is_known flag so
  both known contacts and unknown nodes are shown with useful labels
- getDiscoveredNodes → getDiscoverResults, capacity 8 → 16
- onControlDataRecv populates name from contacts for known nodes and
  records all respondents (not just unknowns)
- NearbyScreen gains _discover_mode sub-screen: 8-second scan window,
  live result polling each render cycle, UP/DOWN scroll, CANCEL to
  return, ENTER/M to re-scan; known nodes show actual name + "known",
  unknown nodes show "[Type]" + "NEW"
- List view is clean: no discovered nodes mixed in, no scanning banner

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

examples/companion_radio/MyMesh.h

@@ -85,10 +85,11 @@ struct AdvertPath {
   uint8_t path[MAX_PATH_SIZE];
 };
 
-struct DiscoveredEntry {
-  uint8_t pub_key_prefix[4];
+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
-  uint32_t timestamp; // RTC timestamp of discovery
+  bool    is_known;   // true = in contacts[], false = new unknown node
+  uint32_t timestamp;
 };
 
 #define EXPECTED_ACK_TABLE_SIZE 8
@@ -111,7 +112,7 @@ public:
   void enterCLIRescue();
 
   int  getRecentlyHeard(AdvertPath dest[], int max_num);
-  int  getDiscoveredNodes(DiscoveredEntry dest[], int max_count);
+  int  getDiscoverResults(DiscoverResult dest[], int max_count);
 
 protected:
   float getAirtimeBudgetFactor() const override;
@@ -272,9 +273,9 @@ 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;
+  #define DISCOVER_RESULTS_MAX 16
+  DiscoverResult  _discover_results[DISCOVER_RESULTS_MAX];
+  int             _discover_count;
   uint32_t        _pending_node_discover_tag;
   unsigned long   _pending_node_discover_until;
 };