Fix subtitles filter path escaping on Windows

[samlwg76]

Problem

On Windows, every render that burns subtitles fails:

[Parsed_subtitles_0 @ ...] Unable to open C:UsersmeprojecteditmasterFsrt
[AVFilterGraph @ ...] Error initializing filters
Error : No such file or directory

Repro: python helpers/render.py edit/edl.json -o final.mp4 --build-subtitles on Windows. Segment extraction, concat and SRT generation all succeed; the failure is in the final composite pass.

Cause

build_final_composite escapes the drive colon but leaves the Windows path separators alone:

subs_abs = str(subtitles_path.resolve()).replace(":", r"\:").replace("'", r"\'")

ffmpeg's filtergraph parser treats \ as an escape character, so the backslashes are consumed before the subtitles filter ever sees them. C:\Users\me\edit\master.srt arrives as C:Usersmeeditmaster.srt.

Fix

Convert to forward slashes before escaping the colon. ffmpeg accepts forward slashes in filter paths on Windows, and the added replace is a no-op on POSIX, where the path contains no backslashes to begin with.

Testing

• Windows 11, ffmpeg 8.1.1, Python 3.12 — --build-subtitles now renders end to end: 7 cues burned into a 1920x1080 final.mp4, loudnorm pass included. Verified the escaped filtergraph string opens the file by running the generated ffmpeg command directly both before and after the change.
• The change is string-level and platform-agnostic; on POSIX str(Path.resolve()) yields no backslashes, so the behaviour is byte-identical to before.

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Summary by cubic

Fixes subtitle burn-in failures on Windows by normalizing subtitle paths in the final composite filtergraph. Backslashes are converted to forward slashes before escaping the drive colon and quotes, so ffmpeg reads the SRT path correctly; POSIX is unaffected.

^{Written for commit 75d6b76. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file

_{Re-trigger cubic}