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/Models.swift

@@ -75,7 +75,7 @@ public struct User: Sendable, Equatable, Identifiable {
     }
 }
 
-/// Permission bitset — mirrors `vc_permissions` (M5).
+/// Permission bitset — mirrors `vc_permissions`.
 public struct Permissions: Sendable, Equatable {
     public let canCreateTempChannel: Bool
     public let canKick: Bool
@@ -91,7 +91,7 @@ public struct Permissions: Sendable, Equatable {
     }
 }
 
-/// Account entry — mirrors `vc_account` (M5, reply to `listAccounts()`).
+/// Account entry — mirrors `vc_account` (reply to `listAccounts()`).
 public struct Account: Sendable, Equatable {
     public let username: String
     public let isAdmin: Bool