fix(cli): reject unsupported MP4 encoders

[miguel-heygen]

Summary

MP4 renders now stop at capability preflight when the selected FFmpeg build has neither libx264 nor the supported VideoToolbox fallback. Previously, the CLI treated this definitive capability result as advisory and continued until the encoder rejected -x264-params; users now get both a distro-aware install command and an actionable --format webm alternative before frame capture starts. Inconclusive probe failures remain advisory, and the existing VideoToolbox fallback is unchanged.

Authorized originating source: https://slack.com/archives/C0BGC335AQY/p1784695521573939

Test plan

• bun run --cwd packages/cli test -- src/browser/ffmpeg.test.ts src/commands/render.test.ts (75 passed)
• bun run --cwd packages/cli typecheck
• bunx oxlint and bunx oxfmt --check on changed files
• bun run build
• Exact fake-FFmpeg fixture: exit 1 at preflight; install/WebM hint present; no x264-params, producer pipeline, or MP4 output

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