feat(backdrop): Capture Window… in-app window pin (TASK 38, Good Dog ONE test)

Click-to-pin an open window as the full-bleed Live backdrop, via the existing
window:<hwnd> capture path (same render layer as game/desktop). Session-scoped:
window:/picker: keys heal to auto on reload (HWND-recycle hazard); pin wins
while the window enumerates, then auto-fallbacks (game → desktop → static); a
dead window's capture session heals to auto, no dead ends.

Fix: ScreenCaptureSourceFactory.Resolve hex-parsed "window:0x<Hwnd>" WITHOUT
stripping the 0x prefix — NumberStyles.HexNumber rejects it, so every pin
resolved null ("No capture target for this key") and the heal silently rolled
back. Same flaw in IsAliveWindowPin (pin-wins guard was dead). Both fixed.

Commit also carries an out-of-scope prerequisite: PillRadioTests.cs:105 had a
committed stray token ("...PrimaryStartButtonLabel soil;", CS1003) blocking the
whole test-project compile — token removed.

Plus the previous session's uncommitted pricing docs (one-time $29/$49) that
share TASKS.md/ai.md/HANDOFF.md.

Services/IWindowEnumerator.cs

@@ -0,0 +1,15 @@
+using System.Collections.Generic;
+using ytLive.Models;
+
+namespace ytLive.Services;
+
+/// <summary>Session-scoped seam that lists open top-level windows for the
+/// "Capture Window…" submenu. Mirrors <see cref="IFullScreenDetector"/> so the
+/// VM never depends on a specific Win32 implementation (and tests hand in a
+/// fake enumerator).</summary>
+public interface IWindowEnumerator
+{
+    /// <summary>Visible, user-facing top-level windows belonging to other
+    /// processes, ordered for display (title, process name).</summary>
+    IReadOnlyList<WindowInfo> Enumerate();
+}