fix(engine): prefer ffmpeg.exe over a .cmd/.bat shim on Windows PATH [P1]

[miguel-heygen]

Root cause

When HYPERFRAMES_FFMPEG_PATH is unset, findOnPath() auto-detects ffmpeg/ffprobe via where (Windows) / which and used the first result. On Windows where ffmpeg frequently lists a .cmd/.bat wrapper (npm/scoop/winget shims) ahead of the real .exe, and Node's spawn without shell:true cannot execute a .cmd/.bat — it throws spawn EINVAL. So render capture/encode and audio muxing failed with "spawn EINVAL" even though FFmpeg was installed.

Two independent reports of this exact EINVAL, one explicitly tracing it to where returning ffmpeg.cmd before ffmpeg.exe (their workaround was renaming the .cmd wrappers so ffmpeg.exe resolved first).

Fix

Extract selectBinaryFromPathResults(output, platform):

• win32: prefer the first directly-spawnable executable (.exe/.com) among the results, falling back to the first result only when none is listed (so a shell-runnable .cmd-only setup isn't dropped).
• non-win32: unchanged (first result).

Scoped to selection. The .cmd-only case (still EINVALs on spawn) is left for a separate shell-spawn change — the common, reported case is both a .cmd shim and a real .exe on PATH, which this fixes.

Test plan

• New selectBinaryFromPathResults unit tests: win32 prefers .exe over a leading .cmd and over a .bat; falls back to the first result when no .exe/.com is listed; returns the first on non-win32; returns undefined for empty output.
• Existing env-resolution tests unchanged; bunx vitest run packages/engine/src/utils/ffmpegBinaries.test.ts — 8/8 passing.
• Full bun run build clean; oxlint/oxfmt clean.

[miguel-heygen]

Closing as superseded — main already landed this exact fix in both resolvers while this branch sat on an older base:

• CLI resolver (packages/cli/src/browser/ffmpeg.ts): fix(cli): prefer real ffmpeg exe over cmd shim #1958 fix(cli): prefer real ffmpeg exe over cmd shim added an inline chooseBestPathCandidate (preferredExe → exact → non-shell-shim → first) used by findOnPath. It's case-insensitive (.toLowerCase()), so @tai's uppercase FFMPEG.EXE nit is already covered. This is the resolver behind snapshot/transcribe/remove-background.
• Engine resolver (packages/engine/src/utils/ffmpegBinaries.ts): b7dcb9e2 fix(engine): harden ffmpeg binary resolution added its own chooseBestPathCandidate + PATH/PATHEXT directory scan.

So the .exe-over-.cmd/.bat preference this PR adds (selectBinaryFromPathResults) is now redundant in both places; nothing here improves on main. The remaining .cmd-only-install EINVAL case @tai flagged is unaddressed on main too and needs the shell-spawn change — tracking that separately rather than reviving this branch.