feat: DualSense BT microphone + USB 3.0 connection watchdog

BT microphone over Bluetooth: the DS5 mic now works over the dongle's BT
pairing — decoded from the controller's Opus stream to the USB capture
endpoint. Hinges on pkt[4] bit 0 (mic-enable) in the outbound 0x36 audio
report; credit to awalol (upstream) for identifying it. Mic-tagged 0x31
frames ((data[2]>>1)&1) are ALWAYS diverted out of the input path (decoded
when on, dropped when off) so Opus payload can never corrupt sticks/buttons.
Always-on via a sticky-latch keep-alive that only runs post-enumeration
(tud_mounted) so it never floods the fresh-pair handshake (which otherwise
delayed controller detection past the watchdog and tore the link down).
Toggle: bt_mic_enable config field (default on) — OLED Settings + web config.
README gains a "DualSense Microphone over Bluetooth" section;
BLUETOOTH_AUDIO_NOTES.md rewritten from "dead end" to the working mechanism.

USB 3.0 connection watchdog: auto-recovers a stalled connection (re-inquiry)
instead of hanging on the amber lightbar, for USB 3.0 ~2.4 GHz RF interference
that desensitizes the CYW43 BT radio. Re-enabled the ACL-fail / auth-fail /
create-connection-reject recovery paths. README "USB 3.0 ports & Bluetooth
interference" section with mitigations.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/config.h

@@ -39,6 +39,11 @@ struct __attribute__((packed)) Config_body {
     // idle timer is 64-bit µs so the full range is representable. Issue #5.
     uint8_t screen_dim_timeout;
     uint8_t screen_off_timeout;
+    // DualSense mic over Bluetooth (Phase I). 0 = off, 1 = on (default). When on,
+    // the dongle asserts the DS5 mic-enable bit so the controller streams its mic
+    // over BT and the dongle decodes it to the USB capture endpoint. Costs extra
+    // DS5 battery (keeps its audio subsystem awake), hence the toggle.
+    uint8_t bt_mic_enable;
 };
 
 struct __attribute__((packed)) Config {