[suharvest]

Problem

Closes #2338.

On networks that drop UDP (e.g. behind a transparent proxy), MQTT signaling works but the UDP audio channel is dead: reply text shows up, no TTS audio plays, and the user's speech never gets an stt.

Solution

• MqttProtocol counts received UDP packets. A tts segment with at least one sentence_start, lasting ≥ 1 s, and zero UDP packets marks UDP as blocked.
• If a WebSocket URL is configured, this is stored per network (AP BSSID) in NVS and the device reboots; InitializeProtocol() then picks WebSocket.
• The fallback lasts 10 boots on that network, then MQTT is retried (period doubles on repeated failure, max 200). A TTS segment that receives UDP audio clears it.
• Kconfig MQTT_UDP_FALLBACK_TO_WEBSOCKET (default y).

Testing

SenseCAP Watcher on a network that drops UDP:

W (32358) MQTT: TTS segment (3 sentences, 7022 ms) delivered no UDP audio
E (32361) MQTT: UDP audio is blocked on this network; rebooting into WebSocket for 10 boots
...
W (5196) MQTT: UDP audio was blocked on this network, using WebSocket (9 boots left)
I (41907) Application: >> 你好小智
I (81459) Application: >> Oh it it.

Builds: sensecap-watcher, bread-compact-ml307.