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)

ytLive.csproj

@@ -24,6 +24,10 @@
     <Resource Include="Assets\llama-logo-icon.png"/>
   </ItemGroup>
 
+  <ItemGroup>
+    <None Include="THIRD-PARTY-NOTICES.txt" CopyToOutputDirectory="PreserveNewest"/>
+  </ItemGroup>
+
   <ItemGroup>
     <AssemblyAttribute Include="System.Runtime.CompilerServices.InternalsVisibleTo">
       <_Parameter1>ytLive.Tests</_Parameter1>