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.cpp

@@ -274,7 +274,7 @@ namespace {
         ue->set_kind(voicecat::v1::UserEvent::LEFT);
         ue->mutable_user()->set_id(user_id);
         ue->set_left_id(user_id);
-        ue->set_reason(reason);  // M5 additive field
+        ue->set_reason(reason);
         return env;
     }
 }
@@ -447,7 +447,7 @@ bool SessionRegistry::check_channel_password(uint32_t channel_id,
     return db_->check_channel_password(channel_id, password);
 }
 
-// ── M2: UDP / media ───────────────────────────────────────────────────────────
+// ── UDP / media ────────────────────────────────────────────────────────────────
 
 void SessionRegistry::register_udp_token(const std::array<uint8_t, 16>& token,
                                           uint64_t session_id) {