chore: comment cleanup pass ahead of open-sourcing

Removes leftover debug scaffolding (stray Console.WriteLine/NSLog traces,
dead nick_buf_ptr, a no-op --print-config flag now implemented for real),
fixes stale/misleading comments (channel passwords are no longer a "future
M5+" feature, a wrong cross-reference, a stale TlsContext::close() mention,
an incomplete BanRecord::subject_type doc, and a smoke test pointing at a
build/m1-dev preset that no longer exists), strips internal M1-M5 milestone
jargon from comments now that the roadmap is done, trims comments that just
restated the following line, and consolidates a few "why" explanations that
were duplicated 2-3 times in the same file.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

core/src/session/session.h

@@ -46,7 +46,7 @@ struct Stream {
     int         kind{0};
     std::string label;
     // Full effective AudioConfig (docs/protocol.md §5), as broadcast by the server in
-    // StreamInfo.audio — mirrors voicecat::v1::AudioConfig field-for-field so M3 per-channel
+    // StreamInfo.audio — mirrors voicecat::v1::AudioConfig field-for-field so per-channel
     // tuning (mono/stereo, bitrate, FEC/DTX, application) is observable client-side, not just
     // sample_rate/frame_ms.
     uint32_t    sample_rate{48000};