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>

src/RemSound.App/RemSound.App.csproj

@@ -14,7 +14,7 @@
          tag_name on the latest GitHub release; bump it on every public release. The
          AssemblyVersion / FileVersion default to this value, and Assembly.GetName().Version
          is what the About dialog and the updater both read. -->
-    <Version>3.9.1</Version>
+    <Version>3.9</Version>
   </PropertyGroup>
 
   <ItemGroup>