Release v3.9: silent-mic fix, buffer drain, idle-send guard, start-up sound, command-line

- Receiver: a plain (Mixed) stream now renders on an active lane when the
  receiver is in two-lane (ASIO) mode, instead of being decoded into a ring
  nothing reads. Fixes one-way silence ("my mic works for me but not for them").
- Receiver: drift resampler gains a buffer-depth correction term so a bloated
  standing buffer eases back to the latency target over a long session.
- App: don't send audio until a peer is genuinely reachable (issue #8); status
  no longer shows phantom send traffic with nobody connected.
- App: start-up cue sound (machine-wide toggle + custom path in Preferences).
- App: command-line options (CommandLine.cs) -- --devices, --selftest,
  --diagnostics, --log, --close, --profile, --connect, --minimized, --version,
  --help. New "Command-line options" manual section (readme.html + MANUAL.md).
- Version 3.9; About-dialog and RELEASE_NOTES updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -24,6 +24,7 @@ It works with a simple shared password. You and the person you're connecting to
 * **Play music together over the internet.** Two musicians at different houses can play along together with very low delay. Much faster than a video call, fast enough that timing-sensitive playing works.
 * **Send a finished mix to a producer or client** in real time, without uploading a file and waiting.
 * **Record what comes through the connection** to WAV, MP3, OGG-Opus, or FLAC. Save sessions for review later.
+* **Drive it from the command line.** As well as its normal window, RemSound takes command-line options — list your audio devices, run a self-test of the whole capture-to-playback path, write a diagnostics report to send for help, or start straight into a profile from a shortcut or script. See [Command-line options](MANUAL.md#22-command-line-options) in the manual.
 
 ## Three quality settings, simple choice