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.

.idea/deploymentTargetSelector.xml

@@ -4,7 +4,7 @@
     <selectionStates>
       <SelectionState runConfigName="app">
         <option name="selectionMode" value="DROPDOWN" />
-        <DropdownSelection timestamp="2026-07-01T14:14:38.599482500Z">
+        <DropdownSelection timestamp="2026-08-05T15:07:39.343414600Z">
           <Target type="DEFAULT_BOOT">
             <handle>
               <DeviceId pluginId="PhysicalDevice" identifier="serial=56211FDAP0015L" />