FFmpeg locator (TASK 4 ship step 2) + licensing: notices, About, LGPL-shared pin

IFfmpegLocator seam that resolves ffmpeg.exe (PATH -> cache -> pinned download),
plus the licensing compliance that makes a paid GA product defensible.

NOTE FOR USERS: this change shows NO difference in the app's behavior except a
new top-bar "About" button (opens THIRD-PARTY-NOTICES.txt). It is scaffolding
for the encoder/streaming work.

- Services/Encoder/: IFfmpegLocator + FfmpegLocator. Pin is BtbN lgpl-shared
  autobuild-2026-08-09-13-03 (NOT gyan.dev/GPL or static: LGPLv2.1 §6 static
  relink material avoided by dynamic linking); extracts ffmpeg.exe + libav*.dll
  via a staging dir so a crash never leaves a partial cache
- THIRD-PARTY-NOTICES.txt: LGPL/BSD/MIT notices + source offer, copied to the
  build output, surfaced by the About button; v1 gate = bundle full license
  texts (TASK 4 requirement 9)
- ai.md "Licensing - do not violate" guardrails (never GPL/nonfree/static/latest-
  tag, never link FFmpeg in, never drop notices); TASKS.md + Services/index.md
  updated
- Tests: FfmpegLocatorTests - hermetic decision-ladder integration test + shared-
  build DLL extraction + edge cases; docs updated (78 tests passing, 0 warnings)

Services/Encoder/IFfmpegLocator.cs

@@ -0,0 +1,20 @@
+namespace ytLive.Services.Encoder;
+
+/// <summary>
+/// Resolves an absolute path to a usable <c>ffmpeg.exe</c>, downloading it on
+/// first use if neither the user's PATH nor the local cache provides one — the
+/// encoder's one external dependency is never shipped in the repo (TASK 4 ship
+/// step 2; see TASKS.md). Seam so the encoder step and the tests can fake it.
+/// </summary>
+public interface IFfmpegLocator
+{
+    /// <summary>
+    /// Returns the path to <c>ffmpeg.exe</c>: the first PATH candidate that
+    /// exists, else the cached copy, else a freshly downloaded one (ffmpeg.exe +
+    /// its libav DLLs extracted from the pinned BtbN LGPL-shared zip into the
+    /// tools directory).
+    /// </summary>
+    /// <exception cref="IOException">The download/extract produced no usable
+    /// binary (offline, expired pin, corrupt archive) — recoverable, logged.</exception>
+    Task<string> LocateAsync(CancellationToken cancellationToken = default);
+}