docs: condense implementation comments

docs/voice.md

@@ -281,6 +281,21 @@ Each receiver keeps an **adaptive jitter buffer per ssrc** with **bounded-depth
   and **Advanced** (every knob manual). A2DP output requires an internal-mic preset (the Bluetooth
   device is output-only); the Stereo/Mono Mic presets fall back to the built-in speaker when no
   external output is connected (`applyA2dpSpeakerFallback`).
+- **iOS implementation invariants:**
+  - External playback is enabled before connecting because authentication can start the audio
+    engine before the UI receives another turn.
+  - AVAudioEngine callbacks may contain multiple codec frames. The mic path writes them to an
+    SPSC ring and releases complete 20 ms frames at a steady cadence; it never consumes a partial
+    frame, and the pacer is recreated when mono/stereo capture changes.
+  - `AVAudioSession` setters can synchronously emit route-change notifications. Configuration is
+    re-entrancy guarded, and recovery ignores `.categoryChange`, `.routeConfigurationChange`, and
+    `.override` because those reasons are generated by the app's own routing calls. External route
+    changes and engine-configuration notifications still rebuild the graph.
+  - Stereo capture anchors the built-in mic's stereo data source. It does not call
+    `setPreferredInputNumberOfChannels(2)`, which can disrupt A2DP output; the core receives the
+    channel count through `vc_set_capture_channels`.
+  - The A2DP speaker fallback caches its last output override. Reapplying the same override would
+    emit another `.override` notification and recursively trigger recovery.
 - **DSP engine: see §11.** The original plan was `webrtc-audio-processing` (AEC + NS + AGC +
   VAD in one tuned module, BSD-licensed) — but it has no working Windows/MSVC build upstream
   (confirmed via its own issue tracker: GCC-only Meson build, MinGW support unfinished, hard