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.

app/src/main/res/values/strings.xml

@@ -65,6 +65,12 @@
     <string name="conn_bluetooth_phase3_desc">Bluetooth connection is planned for Phase 03 and is not yet implemented.</string>
     <string name="conn_esp32_title">ESP32-C5 (USB Serial)</string>
     <string name="conn_esp32_phase3_desc">USB-serial connection to the ESP32-C5 is scaffolded for Phase 03 but not yet functional — it needs the ESP32 firmware protocol translated to Kotlin first.</string>
+    <string name="conn_esp32_state_disconnected">Not connected — plug the ESP32-C5 into the native USB-C port and tap Connect.</string>
+    <string name="conn_esp32_state_device_attached">Device detected, opening…</string>
+    <string name="conn_esp32_state_permission_requested">Waiting for USB permission…</string>
+    <string name="conn_esp32_state_connected">Connected</string>
+    <string name="conn_esp32_state_error">Connection error — check the cable and native USB-C port, then try again.</string>
+    <string name="conn_esp32_connect">Connect to ESP32-C5</string>
     <string name="conn_scan">Scan for Devices</string>
     <string name="conn_scanning">Scanning…</string>
     <string name="conn_usb_title">USB-C (Primary)</string>
@@ -164,7 +170,11 @@
     <string name="settings_obu_hardware">OBU Hardware</string>
     <string name="settings_obu_hardware_cit_one">CiT One</string>
     <string name="settings_obu_hardware_esp32">ESP32-C5</string>
-    <string name="settings_obu_hardware_esp32_note">ESP32-C5 support is scaffolded for Phase 03 but not yet functional. CAM generation moves to the phone; USB-serial connection and DENM trigger are placeholders pending firmware protocol translation.</string>
+    <string name="settings_obu_hardware_esp32_note">ESP32-C5 acts as a "dumb" transceiver: CAM is built and encoded on the phone, sent to the ESP32 over USB serial, and broadcast over ITS-G5. No MQTT broker or DENM use-case engine on this path — see the V2X Monitor screen\'s CAM Pinger for a manual test tool.</string>
+    <string name="settings_esp32_rx_mode">CAM Reception</string>
+    <string name="settings_esp32_rx_mode_send_only">Send Only</string>
+    <string name="settings_esp32_rx_mode_send_and_receive">Send &amp; Receive</string>
+    <string name="settings_esp32_rx_mode_desc">"Send Only" ignores CAM received from nearby stations (still physically received by the ESP32 — its receiver can\'t be turned off without also breaking transmit — just not processed by the app). Useful for isolating TX-path testing.</string>
     <string name="settings_usb_transport">Active transport</string>
     <string name="settings_transport_usbc">USB-C</string>
     <string name="settings_transport_wifi">Wi-Fi</string>
@@ -193,6 +203,17 @@
     <string name="mqtt_denm_show">Show last TX</string>
     <string name="mqtt_denm_hide">Hide</string>
 
+    <!-- CAM Pinger — ESP32-C5-only manual bench test, equivalent to the DENM TX card above -->
+    <string name="mqtt_cam_pinger_title">CAM Pinger (Manual Test)</string>
+    <string name="mqtt_cam_pinger_desc">Fixed-location 1 Hz CAM ping — verifies the serial link and ESP32 radio path without needing GNSS movement or a trip recording.</string>
+    <string name="mqtt_cam_pinger_not_connected">Connect the ESP32-C5 to enable the CAM pinger</string>
+    <string name="mqtt_cam_pinger_active">Pinging — 1 CAM/s over the serial link</string>
+    <string name="mqtt_cam_pinger_sent_count">Sent: %1$d</string>
+    <string name="mqtt_cam_pinger_send_failures">Write failures: %1$d consecutive — CAMs are not reaching the ESP32</string>
+    <string name="mqtt_cam_pinger_fw_counters">ESP32: tx fail %1$d · oversize %2$d · crc err %3$d</string>
+    <string name="mqtt_start_pinger">Start Pinger</string>
+    <string name="mqtt_stop_pinger">Stop Pinger</string>
+
     <!-- CAM-based Use Case Alert panel -->
     <string name="mqtt_usecase_panel_title">CAM Use Case Alerts</string>
     <string name="mqtt_usecase_not_connected">Connect to the OBU to evaluate CAM-based use cases</string>