Phase 03: ESP32-C5 serial link hardening + bench diagnostics

Enumeration:
- Merge the library's stock probe table instead of replacing it, so adding
  Espressif 0x303A/0x1001 doesn't drop every other supported device
- Select the ESP32-C5 by VID/PID rather than list position
- Log USB interface descriptors to distinguish CDC data from the JTAG interface
Lifecycle:
- Don't close the shared port in MqttViewModel.onCleared() - the foreground
  recording service outlives the ViewModel and would beacon into a dead port
- Handle ACTION_USB_DEVICE_DETACHED so the UI stops reporting a stale link
- Implement the STATUS heartbeat on both sides (1 Hz) plus a phone-side watchdog
- Surface write failures and firmware drop counters on the CAM Pinger card
Protocol:
- Assert DTR/RTS on open (unverified on hardware - see FLASHING.md step 5)
- Raise SERIAL_LINK_MAX_PAYLOAD 160 -> 512 on both sides; real third-party CAMs
  exceed 160 and were being silently dropped at the resync branch
- Move the enlarged buffers off task stacks; serialize send_frame with a mutex
Firmware and app must be updated together - a 512/160 mismatch fails silently.

obu-firmware/main/CMakeLists.txt

@@ -5,12 +5,14 @@
 # phone and sent down over serial_link, so this firmware never encodes CAM itself. cam.c/.h are
 # left on disk as the byte-exact reference the Kotlin encoder was ported from - do not delete.
 #
-# serial_link.c/.h - binary phone<->ESP32 UART framing (Phase 03).
+# serial_link.c/.h - binary phone<->ESP32 framing over the native USB Serial/JTAG port
+#                    (Phase 03; changed from a GPIO UART1 wire to native USB because neither
+#                    USB-C port on the ESP32-C5-WIFI6-KIT was actually routed to that UART).
 # gn_unwrap.c/.h - strips 802.11/LLC-SNAP/GeoNetworking/BTP-B off received frames down to CAM
 #                  UPER bytes, for forwarding to the phone over serial_link.
 idf_component_register(
     SRCS "main.c" "denm.c" "geonet.c" "dot11p.c" "tx_custom.c" "serial_link.c" "gn_unwrap.c"
     INCLUDE_DIRS "."
-    REQUIRES esp_event esp_netif nvs_flash driver esp_phy
+    REQUIRES esp_event esp_netif nvs_flash driver esp_phy esp_driver_gpio esp_driver_usb_serial_jtag
     PRIV_REQUIRES esp_wifi
 )