Add YD-ESP32-S3 support, boot diagnostics, and hang guards

Board is now selected by `default_envs` in platformio.ini rather than being
hardcoded. Both boards stay supported and both targets build.

- src/board_pins.h holds one block per board plus an ledSet() abstraction, so
  main.cpp has no board-specific code. The C6's active-low LED and the S3's
  WS2812 RGB go through the same call, and the C6 antenna-switch lines are
  behind #if HAS_RF_SWITCH.
- YD-ESP32-S3 env targets esp32-s3-devkitc1-n16r8 (verified against the actual
  part: ESP32-S3 rev v0.2, 16MB quad flash, 8MB octal PSRAM, CH343 bridge).
  Serial is UART0 -> CH343, not USB CDC.

Fixes for hangs that left the board alive on USB but dead on the network:

- The vendor library's UART path has loops with no timeout: sensorStop() is
  while(1) that only exits on a "sensorStop" reply, and the UART branch of
  getStatus() is while(len == 0). Calling either with no sensor attached hung
  setup() permanently. UART is now probed with a raw 1.5s read on Serial1, and
  the library only gets the port once bytes actually arrive. The I2C path is
  bounded, so prefer I2C.
- Floating SDA/SCL can stall Wire.endTransmission() forever. Check both lines
  read high with pull-ups before touching Wire.
- Serial.setTxTimeoutMs(0) is a trap: HWCDC::write seeds a uint32_t from it and
  does a bare tries--, so 0 underflows to 4.29e9 and spins for ~49 days. Use 10,
  and only on USB CDC boards.

Diagnostics, so "did it crash?" is answerable without a serial cable:

- Boot count in RTC memory, esp_reset_reason(), free/min heap and WiFi drop
  count exposed via /api/live and shown as header pills in the UI.
- WiFi auto-reconnect, and retry of station mode from the AP fallback. Without
  it a single dropout left the board unreachable forever, which looks exactly
  like a crash.

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

platformio.ini

@@ -1,28 +1,51 @@
-; XIAO ESP32-C6 + DFRobot C4001 24GHz mmWave presence sensor
+; XIAO ESP32-C6 / YD-ESP32-S3 + DFRobot C4001 24GHz mmWave presence sensor
 ;
-; Board detected on this machine:
-;   ESP32-C6FH4 (QFN32) rev v0.1, 4MB embedded flash, USB-Serial/JTAG
-;   MAC e4:b3:23:b4:7b:c4  on /dev/ttyACM0
-;
-; NOTE: ESP32-C6 needs Arduino-ESP32 core 3.x. The installed platform
-; (espressif32 @ 2026.02.30, the pioarduino fork) ships core 3.3.7 and has
-; the XIAO_ESP32C6 variant, so this builds as-is. The old espressif32 6.x
-; from PlatformIO's registry does NOT support C6 -- don't downgrade.
+; ============================================================================
+;  TO SWITCH BOARDS: change `default_envs` below to one of the [env:] names.
+;    yd_esp32_s3   -> YD-ESP32-S3 (ESP32-S3-WROOM-1 N16R8), CH343 COM port
+;    xiao_esp32c6  -> Seeed XIAO ESP32-C6, native USB Serial/JTAG
+;  Pin maps for each live in src/board_pins.h.
+;  Or build one explicitly without editing this file:  pio run -e xiao_esp32c6
+; ============================================================================
 
-[env:seeed_xiao_esp32c6]
-platform  = espressif32
-board     = seeed_xiao_esp32c6
+[platformio]
+default_envs = yd_esp32_s3
+
+; ---------------------------------------------------------------- shared ----
+[env]
 framework = arduino
-
 monitor_speed   = 115200
 monitor_filters = esp32_exception_decoder, time
-upload_port     = /dev/ttyACM0
-monitor_port    = /dev/ttyACM0
-
-; Pulled straight from DFRobot's repo. The registry copy lags behind.
 lib_deps =
+    ; Registry copy lags behind; take it straight from DFRobot.
     https://github.com/DFRobot/DFRobot_C4001.git
 
+; ----------------------------------------------------- YD-ESP32-S3 N16R8 ----
+; Detected on this machine: ESP32-S3 (QFN56) rev v0.2, 16MB quad flash,
+; 8MB octal PSRAM, MAC 24:58:7c:cd:9a:d8, CH343 bridge (1a86:55d3).
+;
+; The board has two USB-C sockets. This env assumes the one marked COM (the
+; CH343 UART bridge). Serial is therefore real UART0, not USB CDC, which also
+; sidesteps the CDC-blocking behaviour that bit us on the C6.
+[env:yd_esp32_s3]
+platform = espressif32
+board     = esp32-s3-devkitc1-n16r8
 build_flags =
-    -DARDUINO_USB_CDC_ON_BOOT=1   ; Serial -> USB CDC, so the monitor works over the native port
+    -DBOARD_YD_ESP32_S3
+    -DARDUINO_USB_CDC_ON_BOOT=0   ; Serial -> UART0 -> CH343 -> /dev/ttyACM0
+    -DCORE_DEBUG_LEVEL=1
+
+; ------------------------------------------------------ Seeed XIAO ESP32-C6 --
+; Detected earlier: ESP32-C6FH4 (QFN32) rev v0.1, 4MB flash,
+; MAC e4:b3:23:b4:7b:c4, native USB Serial/JTAG (303a:1001).
+;
+; Needs Arduino-ESP32 core 3.x. The installed platform is the pioarduino fork
+; (2026.02.30, core 3.3.7) which has the XIAO_ESP32C6 variant. The old registry
+; espressif32 6.x has no C6 support -- don't downgrade.
+[env:xiao_esp32c6]
+platform = espressif32
+board     = seeed_xiao_esp32c6
+build_flags =
+    -DBOARD_XIAO_C6
+    -DARDUINO_USB_CDC_ON_BOOT=1   ; Serial -> native USB CDC
     -DCORE_DEBUG_LEVEL=1