Fix subtitle path escaping on Windows in render.py

[elextechnico-dotcom]

Summary

• On Windows, render.py's subtitle compositing step fails with an ffmpeg error like `Unable to open C:appsprojectedit master.srt` — the drive-letter colon and backslashes get mangled before reaching ffmpeg.
• Root cause: ffmpeg's own filtergraph parser treats \ as its escape character, so it consumes the path's backslashes before the code's own .replace(":", r"\:") colon-escaping can take effect. A raw Windows path like C:\apps\project\edit\master.srt ends up losing its backslashes and the colon escape never lands where it should, producing a broken path.
• Fix: normalize to forward slashes first (.replace("\\", "/")), which ffmpeg accepts natively on Windows, before applying the existing colon/quote escaping. This sidesteps the backslash-escaping problem entirely instead of trying to double-escape it.

Found and fixed while using this tool for a real edit session on Windows (verified via a full render with subtitles + overlays, output confirmed correct via ffprobe and visual inspection).

Test plan

• python -m ast / syntax check on the modified file
• Full render.py run on Windows with subtitles set in the EDL — subtitle filter now resolves the path correctly and the render completes
• Verified output video has burned-in subtitles matching the source .srt, correctly timed

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Summary by cubic

Fixes subtitle path escaping on Windows in render.py so ffmpeg's filtergraph parser no longer mangles the path and renders complete with correct burned-in subtitles.

Bug Fixes

• Normalizes Windows paths to forward slashes before applying colon and quote escaping, sidestepping ffmpeg's backslash escape handling.

^{Written for commit 3ddf20b. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file

_{Re-trigger cubic}