fix(render): fall back when libx264 is unavailable

[miguel-heygen]

Summary

• inspect the selected FFmpeg build before local MP4 rendering
• automatically enable the supported H.264 hardware path when libx264 is absent
• preserve the software path when libx264 is available and surface a clear warning on fallback

Regression

A macOS FFmpeg 8.1.2 build exposing h264_videotoolbox but not libx264 failed the default high-quality render with Unrecognized option 'preset'; --gpu succeeded. The new tests model that capability list and assert the render job switches to hardware encoding.

Verification

• bun --cwd packages/cli vitest run src/browser/ffmpeg.test.ts src/commands/render.test.ts (58 passed)
• cd packages/cli && bun run typecheck
• pre-commit lint/format/fallow/typecheck hooks passed

Source: https://heygen.slack.com/archives/C0BGC335AQY/p1783985360149839

[miguel-heygen]

Consolidated with the related render preflight and recovery fixes in #2403. The original behavior and applicable regression coverage are preserved there.