perf: run yt-dlp from a zipapp on a portable Python

Replaces the official yt-dlp_macos sidecar. That build is PyInstaller
one-file and unpacks 37MB on every invocation, costing ~8s per call on
this machine — ruled out signing, xattrs and thinning the universal
binary as causes, and it is 8% CPU over 8s, so it is the unpack itself.
Every stream resolve paid it.

The 3MB zipapp on a trimmed portable CPython starts in ~0.45s, a 17x
improvement; a real stream resolve goes from ~10s to ~2.4s. macOS ships
only Python 3.9 and yt-dlp needs 3.10+, hence bundling an interpreter.

yt-dlp is now resolved as an argv prefix rather than a path, so a system
install (a plain script, instant) still takes precedence.

Bundle grows 153MB -> 219MB, which buys back the responsiveness.

.gitignore

@@ -28,5 +28,7 @@ dist-ssr
 *.sln
 *.sw?
 
-# Bundled helper binaries — ~140MB, fetched by scripts/fetch-binaries.sh
+# Bundled helper tools — ~130MB, fetched by scripts/fetch-binaries.sh
 src-tauri/binaries/
+src-tauri/resources/python/
+src-tauri/resources/yt-dlp.pyz