fix(windows): real native exclude + self-echo removal for screen audio

"All apps except selected" previously captured the complement of a frozen
app snapshot in INCLUDE mode (missed late-launched apps and system sounds,
wasted captures on silent windows). It now opens a single ProcessLoopbackCapture
in EXCLUDE mode (AUDIOCLIENT_PROCESS_LOOPBACK_MODE_EXCLUDE_TARGET_PROCESS_TREE)
of the one chosen app — true system-mix-minus-one, dynamic so apps launched
after sharing starts are included. The picker enforces single-selection in
exclude mode (the activation params take one target PID).

Adds an "Exclude VoiceCat's own audio (prevents echo)" checkbox (default on,
entire-desktop only) that routes the desktop capture through the same EXCLUDE
path targeting our own process id, killing the whole-device self-echo loop.

No C++/ABI changes. Updates voice.md and PROGRESS.md.

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

clients/windows/VoiceCat.App/Audio/AudioSessionEnumerator.cs

@@ -6,7 +6,10 @@ namespace VoiceCat.App.Audio;
 // ── Scope types (mirror macOS ScreenAudioScope / ScreenAudioSelection) ────────
 
 public abstract record AppAudioScope;
-public sealed record EntireDesktop : AppAudioScope;
+// ExcludeSelf=true captures the whole system render mix minus VoiceCat's own process tree
+// (kills the self-echo loop). Routed through the external-feed mixer as a single EXCLUDE
+// capture; ExcludeSelf=false keeps the core's whole-device loopback path.
+public sealed record EntireDesktop(bool ExcludeSelf = false) : AppAudioScope;
 public sealed record OnlyApps(IReadOnlyList<int> Pids, IReadOnlyList<string> Names) : AppAudioScope;
 public sealed record AllExceptApps(IReadOnlyList<int> Pids, IReadOnlyList<string> Names) : AppAudioScope;