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>

clients/apple/Sources/VoiceCatCore/VoiceCatClient.swift

@@ -219,7 +219,7 @@ public final class VoiceCatClient {
         VoiceCatResult(vc_authenticate_user(handle, username, password))
     }
 
-    // MARK: - TOFU server-identity gate (M4)
+    // MARK: - TOFU server-identity gate
 
     /// Accept or reject the pending server-identity check. Call after a `.serverIdentity`
     /// event. `accept=true` on firstConnect/mismatch updates the pin file and proceeds;
@@ -413,7 +413,7 @@ public final class VoiceCatClient {
 
     /// Global playback volume applied after mixing all remote streams. gain 0.0 = silent,
     /// 1.0 = unity (default), >1.0 amplifies. Always LOCAL — no protocol traffic. Mirrors the
-    /// Windows client's `SetOutputVolume` and the C ABI `vc_set_output_volume` added in M5.
+    /// Windows client's `SetOutputVolume` and the C ABI `vc_set_output_volume`.
     @discardableResult
     public func setOutputVolume(_ gain: Float) -> VoiceCatResult {
         VoiceCatResult(vc_set_output_volume(handle, gain < 0 ? 0 : gain))
@@ -500,7 +500,7 @@ public final class VoiceCatClient {
         return Marshaling.devices(&native)
     }
 
-    // MARK: - M5: Moderation
+    // MARK: - Moderation
 
     @discardableResult
     public func kickUser(_ userId: UInt32, reason: String? = nil) -> VoiceCatResult {
@@ -535,7 +535,7 @@ public final class VoiceCatClient {
         VoiceCatResult(vc_move_user(handle, userId, channelId))
     }
 
-    // MARK: - M5: Channel admin
+    // MARK: - Channel admin
 
     @discardableResult
     public func createChannel(_ info: ChannelEdit) -> VoiceCatResult {
@@ -558,7 +558,7 @@ public final class VoiceCatClient {
         VoiceCatResult(vc_delete_channel(handle, channelId))
     }
 
-    // MARK: - M5: Account admin
+    // MARK: - Account admin
 
     @discardableResult
     public func createAccount(_ username: String, password: String) -> VoiceCatResult {