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/CMakeLists.txt

@@ -6,7 +6,7 @@ file(GLOB_RECURSE VOICECAT_SOURCES CONFIGURE_DEPENDS
 if(VOICECAT_BUILD_SHARED)
     add_library(voicecat SHARED ${VOICECAT_SOURCES})
     if(WIN32 AND MINGW)
-        # M4: the C# client only ships voicecat.dll itself — no MinGW runtime DLLs alongside
+        # The C# client only ships voicecat.dll itself — no MinGW runtime DLLs alongside
         # it. x64-mingw-static only statically links vcpkg's OWN library deps (protobuf,
         # sodium, mbedTLS, ...); the GCC/MinGW runtime stays dynamic by default
         # (libgcc_s_seh-1.dll/libwinpthread-1.dll/libstdc++-6.dll — confirmed via `objdump -p`