[SYS] Migrate M5 boards to M5Unified + M5GFX (modern platform) (#2355)

The per-board libs (M5Stack/M5StickC/M5StickC-Plus/M5Tough) don't build on
Arduino core 3.x, which kept the five M5 BLE envs pinned to espressif32@6.1.0.
Port board support to M5Unified + M5GFX so they run ${com.esp32_platform} like
the rest of the tree.

- environments.ini: 5 M5 envs -> modern platform + m5unified. m5stick-c uses the
  pico32 variant (m5stick_c variant absent upstream) and no_coredump partitions
  (M5Unified overflows min_spiffs).
- boardM5.cpp / config_M5.h: single <M5Unified.h>; M5.Lcd aliases M5.Display;
  unified brightness/sleep/wake.
- main.cpp / mqttDiscovery.cpp: battery telemetry via M5.Power; AXP192-only
  Vin/VBus/temp fields dropped (no portable equivalent).

All five M5 envs build; M5Stack Core hardware-verified.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

main/config_M5.h

@@ -26,18 +26,12 @@
 #ifndef config_M5_h
 #define config_M5_h
 
-#ifdef ZboardM5STICKC
-#  include <M5StickC.h>
-#endif
-#ifdef ZboardM5STICKCP
-#  include <M5StickCPlus.h>
-#endif
-#ifdef ZboardM5STACK
-#  include <M5Stack.h>
-#endif
-#ifdef ZboardM5TOUGH
-#  include <M5Tough.h>
-#endif
+// All M5 boards (Core / Stick C / Stick C Plus / Tough) are driven through the
+// unified M5Unified + M5GFX libraries, which build on the modern Arduino core
+// and auto-detect the board at runtime. This replaces the per-board libraries
+// (M5Stack / M5StickC / M5StickC-Plus / M5Tough) that were pinned to Arduino
+// core 2.x (espressif32@6.1.0) because they don't compile on core 3.x.
+#include <M5Unified.h>
 
 extern void setupM5();
 extern void loopM5();