fix: classify FFmpeg log messages as ffout/fferr instead of always stderr

[PiedPiper911]

Summary

Fixes #877 - ffmpeg.on('log') always reports type: "stderr" regardless of whether the output is an error or informational message.

Root Cause

FFmpeg natively writes virtually ALL its output to stderr -- including version info, stream details, encoding progress, and other informational messages. This is by design in native FFmpeg. So the printErr() callback in bind.js gets called for everything, and every log event gets type: "stderr".

Fix

Modified printErr() in src/bind/ffmpeg/bind.js to classify messages by content:

• "ffout": Informational stderr output (progress bars, stream info, version, etc.)
• "fferr": Actual error messages matching patterns like [error], [fatal], not found, invalid, cannot, etc.
• "stdout": Unchanged (from print())

Breaking Change Note

Users who were checking type === "stderr" will need to update to type === "fferr" for error detection. This is a deliberate API improvement -- the old behavior was the bug itself.

Changes

• src/bind/ffmpeg/bind.js: Added FFMPEG_ERROR_PATTERNS regex array and classification logic in printErr()
• packages/types/types/index.d.ts: Updated Log.type JSDoc to document new values

Test plan

• Run a successful transcoding command - all log events should have type "ffout", never "stderr"
• Run a failing command (e.g., nonexistent input file) - error log events should have type "fferr"
• Verify stdout logs still work unchanged

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[PiedPiper911]

Closing rather than leaving a permanently-red check. The CI here builds the ffmpeg-core through Docker with an exported build cache, which a fork PR can't consume, so this job can't pass for external contributors. The change itself is small (classify log lines as ffout/fferr); happy to resubmit if there's a lighter path for external PRs.