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/proto/voicecat.proto

@@ -122,7 +122,7 @@ message User {
   bool   self_deafened = 6;
   bool   server_muted = 7;
   repeated StreamInfo streams = 8;
-  bool   server_deafened = 9;  // M5: server-imposed deafen
+  bool   server_deafened = 9;  // server-imposed deafen
   bool   voice_subscribed = 10; // true when the user is on the voice plane (hears + can send)
 }
 
@@ -194,7 +194,7 @@ message UserEvent {
   Kind kind = 1;
   User user = 2;
   uint32 left_id = 3;
-  string reason = 4;  // M5: kick/ban reason for LEFT events
+  string reason = 4;  // kick/ban reason for LEFT events
 }
 message SubscribeRequest { repeated uint32 channel_ids = 1; bool presence = 2; }