fix(engine): support current FFmpeg filter scripts

[miguel-heygen]

What

Audio mixing now works on current FFmpeg nightlies without giving up compatibility with older stable installations or reintroducing Windows command-line overflow for large track counts.

Why

FFmpeg removed the deprecated -filter_complex_script alias from nightly builds. HyperFrames therefore emitted a video-only fallback even though the same filter graph is valid through FFmpeg's replacement -/filter_complex <file> syntax. Older FFmpeg builds do not recognize that replacement, so switching unconditionally would break existing users.

How

The mixer keeps the legacy file-valued option as its first attempt and retries with -/filter_complex only when stderr explicitly reports that filter_complex_script is unavailable. Both paths use the same temporary graph file, keeping 100+ track graphs off argv, and cleanup still runs after either attempt.

Test plan

• Reproduced Unrecognized option 'filter_complex_script' on FFmpeg nightly N-125551
• Verified -/filter_complex succeeds on the identical nightly graph
• Verified local FFmpeg 4.2 rejects -/filter_complex, exercising the backward-compatibility requirement
• bun run --cwd packages/engine test -- audioMixer.test.ts (12/12)
• bun run --cwd packages/engine typecheck
• bunx oxlint and bunx oxfmt --check on changed files
• Pre-commit hooks, including typecheck and tracked-artifact checks

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