Service: offer "Use Windows default output" (Christopher's request), reusing the app's follower

The service profile could only pick named output devices, so it couldn't "just send
whatever this machine plays and follow the Windows default". The main app already had
exactly that (its DefaultLoopbackSendFollower + ResolveDefaultDeviceId) - so rather
than invent a parallel mechanism (which would silently diverge), this pulls the shared
piece out and has the service reuse it.

- New AudioDefaultFollower: one home for the loopback-send sentinel
  ("__use-default-loopback-send__"), the follower list entry, and the default-endpoint
  resolver. MainForm now references it (its DefaultLoopbackSendFollower and
  ResolveDefaultDeviceId delegate to it) so there is a single definition.
- Service config dialog: the Audio send tab lists "Use Windows default audio device,
  follows Windows changes" as the first output choice. Ticking it persists the same
  sentinel the app uses.
- ServiceSendHost.BuildSendSpecs resolves that sentinel to the LIVE default render
  endpoint (with de-dup against explicitly-ticked devices), never passing the raw
  sentinel through. Because the service re-applies its profile on every device-change
  notification - and OnDefaultDeviceChanged is one of them - it FOLLOWS the default:
  change Windows' default output and the service switches to it within a beat.
- Manual: documents the new option in the service section.

Test: new "Default-output follower" self-test - follower is flagged + shares the app's
sentinel, and the service resolves it to the current Windows default render endpoint
(never leaks the raw sentinel into a capture spec). Gate 44/44.

The ASIO "Rea" devices Ed noticed are real registry drivers (Realtek ASIO + REAPER's
ReaRoute), not injected dummies - we only ever list HKLM\SOFTWARE\ASIO. No code change.

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

readme.html

@@ -1413,7 +1413,7 @@ RemSound.exe --connect 192.168.1.42
 <h3>Setting it up</h3>
 <p>Everything lives in the <strong>Service</strong> menu on the menu bar:</p>
 <ol>
-<li><strong>Configure service profile&hellip;</strong> &mdash; opens a small window with two tabs (Connectivity and Audio send) where you choose who to send to (plus a password) and what to send. There is no &ldquo;send my audio&rdquo; switch because the service always sends, and there is no audio-quality tab to fiddle with: the service always uses the settings that work best for live streaming (the Opus live-latency codec, small packets, locked to the audio clock), so it just sounds right. This is a separate profile from your normal ones and does not appear in the usual profile list. The <strong>Additional options</strong> button lets you turn the connect/disconnect sounds and the service's own log on or off.</li>
+<li><strong>Configure service profile&hellip;</strong> &mdash; opens a small window with two tabs (Connectivity and Audio send) where you choose who to send to (plus a password) and what to send. On the Audio send tab, the first output choice is <strong>Use Windows default audio device, follows Windows changes</strong> &mdash; tick that to send whatever this machine is currently playing and keep following the Windows default if it later changes, rather than pinning one named card. (You can still pick specific devices, or a specific application, exactly as in the normal app.) There is no &ldquo;send my audio&rdquo; switch because the service always sends, and there is no audio-quality tab to fiddle with: the service always uses the settings that work best for live streaming (the Opus live-latency codec, small packets, locked to the audio clock), so it just sounds right. This is a separate profile from your normal ones and does not appear in the usual profile list. The <strong>Additional options</strong> button lets you turn the connect/disconnect sounds and the service's own log on or off.</li>
 <li><strong>Install service</strong> &mdash; registers it with Windows so it starts automatically at every boot. Windows asks for administrator permission (one prompt). Do this once. (When you first install RemSound on a PC, the installer also offers to set the service up for you, so you may have done this already.)</li>
 <li><strong>Start service</strong> / <strong>Stop service</strong> &mdash; run or halt it now without waiting for a reboot.</li>
 <li><strong>Uninstall service</strong> &mdash; removes it entirely.</li>