feat(audio): mic packet-loss concealment (jitter buffer + Opus PLC)

The BT mic decode path gained a decoded-frame jitter buffer (8 frames) drained
at a steady 10 ms playout cadence. Bursty BT delivery is smoothed; a dropped
mic frame during an active session is concealed with an Opus PLC frame
(opus_decode(decoder, NULL, 0, ...)) instead of leaving a hole the host hears
as a click/dropout. Playout pre-buffers 3 frames and stops after 300 ms of no
real frames so it never emits comfort noise when the mic is idle. New "Mic PLC:"
Diagnostics counter climbs only when concealment fires (a live link-quality
gauge). Verified on hardware: forced BT loss kept captured audio gap-free
(longest zero-run ~0 ms) while the counter climbed; clean link leaves it idle.
Adds ~30 ms mic latency (the pre-buffer). Design ported from
SundayMoments/DS5_Bridge (credit). Unreleased — batching with the next feature.

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

README.md

@@ -157,6 +157,7 @@ This was long believed impossible — earlier versions of this fork documented i
 - Mic audio is **mono** (decoded mono, duplicated across the stereo capture endpoint).
 - Toggling off mid-session stops the host feed immediately, but the controller keeps streaming until it next reconnects (there's no known "stop" command); connecting fresh with the toggle off never enables it.
 - The OLED **Diagnostics** screen's `Mic in:` counter reads ~100/s while the mic is streaming — a quick way to confirm it's live.
+- **Packet-loss concealment:** dropped mic frames (weak BT link, distance, interference) are concealed with Opus PLC so voice stays continuous instead of clicking/cutting out, at a small jitter-buffer latency (~30 ms). The Diag screen's `Mic PLC:` counter climbs only when frames are being concealed — effectively a live link-quality gauge.
 
 ## Known Issues