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.cpp

@@ -125,6 +125,7 @@ static const int QUICK_MSGS_MAX = 10;
 #include "AutoAdvertScreen.h"
 #include "TrailScreen.h"
 #include "CompassScreen.h"
+#include "DiagnosticsScreen.h"
 #include "ToolsScreen.h"
 
 #ifndef BATT_MIN_MILLIVOLTS
@@ -1198,6 +1199,7 @@ void UITask::begin(DisplayDriver* display, SensorManager* sensors, NodePrefs* no
   auto_advert_screen = new AutoAdvertScreen(this, node_prefs);
   trail_screen       = new TrailScreen(this, &_trail);
   compass_screen     = new CompassScreen(this);
+  diag_screen        = new DiagnosticsScreen(this);
   applyBrightness();
   applyFont();
   applyRotation();
@@ -1244,6 +1246,10 @@ void UITask::gotoCompassScreen() {
   setCurrScreen(compass_screen);
 }
 
+void UITask::gotoDiagnosticsScreen() {
+  setCurrScreen(diag_screen);
+}
+
 void UITask::gotoAutoAdvertScreen() {
   ((AutoAdvertScreen*)auto_advert_screen)->enter();
   setCurrScreen(auto_advert_screen);