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>

server/src/session_registry.h

@@ -2,7 +2,7 @@
  * server/session_registry.h — In-memory session, channel, and user registry.
  *
  * Tracks all authenticated sessions, the channel tree, user<→>channel assignments,
- * UDP endpoint bindings (M2), and SSRC<→>session mappings (M2).
+ * UDP endpoint bindings, and SSRC<→>session mappings.
  * Protected by a shared_mutex (many readers, few writers). All methods are thread-safe.
  */
 #ifndef VOICECAT_SERVER_SESSION_REGISTRY_H
@@ -144,7 +144,7 @@ class SessionRegistry {
     // Check a channel password.
     bool check_channel_password(uint32_t channel_id, const std::string& password) const;
 
-    // ── M2: UDP / media ────────────────────────────────────────────────────────
+    // ── UDP / media ────────────────────────────────────────────────────────────
 
     // Register a session's UDP token (called at auth success).
     void register_udp_token(const std::array<uint8_t, 16>& token, uint64_t session_id);
@@ -177,7 +177,7 @@ class SessionRegistry {
     // Return the channel_id of a user (0 if not found).
     uint32_t user_channel(uint32_t user_id) const;
 
-    // Return a channel's authoritative AudioConfig (M3 per-channel Opus tuning), or nullopt
+    // Return a channel's authoritative AudioConfig (per-channel Opus tuning), or nullopt
     // if the channel doesn't exist. There is no per-id Channel getter today otherwise —
     // channel_snapshot() copies every channel, which callers needing just one config should
     // avoid.
@@ -199,7 +199,7 @@ class SessionRegistry {
     std::unordered_map<uint32_t, ChannelEntry>                channels_;
     std::unordered_map<uint64_t, voicecat::v1::Permissions>   session_permissions_;
 
-    // M2: token → session_id (populated at auth, cleared on disconnect)
+    // Token → session_id (populated at auth, cleared on disconnect)
     struct TokenHash {
         size_t operator()(const std::array<uint8_t, 16>& t) const {
             // FNV-1a over 16 bytes
@@ -210,10 +210,10 @@ class SessionRegistry {
     };
     std::unordered_map<std::array<uint8_t, 16>, uint64_t, TokenHash> udp_tokens_;
 
-    // M2: UDP endpoint → session_id (populated after UDP binding packet arrives)
+    // UDP endpoint → session_id (populated after UDP binding packet arrives)
     std::unordered_map<asio::ip::udp::endpoint, uint64_t, UdpEndpointHash> udp_endpoints_;
 
-    // M2: ssrc → session_id (populated when StreamAnnounce is processed)
+    // ssrc → session_id (populated when StreamAnnounce is processed)
     std::unordered_map<uint32_t, uint64_t> ssrc_to_session_;
     std::atomic<uint32_t> next_ssrc_{1};
 };