Stage the plugin into the directory layout OBS actually searches

The staged package put the module in build/package/bin. OBS does not look
there. From AddExtraModulePaths() in obs-studio's UI/window-basic-main.cpp,
the per-user plugin layout on Linux and Windows is:

  <config>/obs-studio/plugins/<name>/bin/64bit/<name>.{so,dll}
  <config>/obs-studio/plugins/<name>/data/

so build/package/ now uses bin/64bit and is a straight drop-in. Re-verified in
the headless libobs harness from the new path: the module loads, both sources
connect, frames arrive, the camera switch round-trips, and `ldd` on the staged
copy resolves liblivekit and liblivekit_ffi from bin/64bit via $ORIGIN.

macOS deliberately keeps the flat bin/ -- there OBS looks for a
<name>.plugin/Contents/MacOS bundle, which this build does not produce. That
gap is documented in README.md rather than papered over with a directory name
that would only look right.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RL8abRmgFXkVASHkkqiJbE

README.md

@@ -109,15 +109,20 @@ and `-DSTPLUGIN_LIVEKIT_SDK_TRIPLE` override the pin and the release triple.
 The build stages a runnable layout into `build/package/`:
 
 ```
-build/package/bin/streamer-tools-camera.so   (RPATH=$ORIGIN)
-build/package/bin/liblivekit.so
-build/package/bin/liblivekit_ffi.so
+build/package/bin/64bit/streamer-tools-camera.so   (RPATH=$ORIGIN)
+build/package/bin/64bit/liblivekit.so
+build/package/bin/64bit/liblivekit_ffi.so
 build/package/data/locale/en-US.ini
 build/package/licenses/...
 ```
 
-`build/package/bin` is what gets installed — the module resolves the LiveKit
-libraries from `$ORIGIN` / `@loader_path`, not from the build tree.
+That is exactly the layout OBS searches on Linux and Windows —
+`<config>/obs-studio/plugins/<name>/bin/64bit` plus a sibling `data/`, per
+`AddExtraModulePaths()` in obs-studio's `UI/window-basic-main.cpp` — so
+`build/package/` is a straight drop-in. The module resolves the LiveKit
+libraries from `$ORIGIN` (verified: `ldd` on the staged copy resolves both
+to `bin/64bit/`), not from the build tree. macOS is not this shape; see the
+macOS packaging gap under CI.
 
 ## Testing this by hand
 
@@ -131,6 +136,10 @@ cp -r build/package/bin build/package/data \
 obs
 ```
 
+(That yields `.../streamer-tools-camera/bin/64bit/streamer-tools-camera.so`
+and `.../streamer-tools-camera/data/locale/en-US.ini`, which is what OBS
+looks for.)
+
 Then: Sources → `+` → "streamer-tools Camera" → fill in the server URL, room
 slug and read key from the room's settings page → "Refresh camera list" → pick
 a camera. Check `~/.config/obs-studio/logs/` for