feat(ui): add on-device diagnostics screen (Tools > Diagnostics)

Shows packet counts by category (RX/TX), radio noise floor/RSSI/SNR,
packet-pool free count and outbound queue length, uptime, and heap/stack
headroom on a single scrollable screen.

Adds generic per-payload-type RX/TX counters to Dispatcher (mesh layer),
plus pool-free/queue-length getters, and a new DeviceDiag helper for
nRF52 heap (linker-symbol + sbrk) and stack (FreeRTOS high-water-mark)
stats — the only platform needed for the 3 in-scope boards (Wio Tracker
L1 OLED/Eink, GAT562 30S), all nRF52840.

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

examples/companion_radio/ui-new/UITask.h

@@ -79,6 +79,7 @@ class UITask : public AbstractUITask {
   UIScreen* auto_advert_screen;
   UIScreen* trail_screen;
   UIScreen* compass_screen;
+  UIScreen* diag_screen;
   UIScreen* curr;
   CayenneLPP _dash_lpp;
   TrailStore _trail;
@@ -152,6 +153,7 @@ public:
   void gotoAutoAdvertScreen();
   void gotoTrailScreen();
   void gotoCompassScreen();
+  void gotoDiagnosticsScreen();
   TrailStore& trail() { return _trail; }
   WaypointStore& waypoints() { return _waypoints; }
   // Shared on-screen keyboard — only one screen drives it at a time.