docs: align subtitle and output guidance with render.py

[hydraxman]

Summary

• update the documented bold-overlay subtitle margin to match SUB_FORCE_STYLE
• clarify render.py's current scaling and frame-rate behavior
• point custom FFmpeg filters to grade.py, which owns the --filter option
• fix the stale subtitle margin in render.py's module documentation

Problem

The editing guide still documents MarginV=35, while the renderer uses MarginV=90 for vertical-video safe zones. It also suggests passing --filter to render.py, but that option is exposed by grade.py instead.

Testing

• python3 -m py_compile helpers/*.py — passed
• python3 helpers/render.py --help — confirmed no --filter option
• python3 helpers/grade.py --help — confirmed --filter is available
• git diff --check — passed

Addresses the documentation drift tracked in #64.

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Summary by cubic

Align docs with current render.py behavior and fix the stale subtitle margin in its docstring. Updated bold-overlay to MarginV=90, clarified render.py renders at 24 fps and scales the long edge to 1920px (1280px in draft), and pointed --filter usage to grade.py.

^{Written for commit 4cd7c04. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}