fix: resolve folder-wide manim.cfg relative to the scene file, not the cwd

[Selahattinozdmr]

Summary

The folder-wide manim.cfg was being located via a bare relative Path("manim.cfg") in config_file_paths(), which configparser resolves against the current working directory. This lookup happens at import time, before the CLI has parsed args.file, so the documented behavior did not hold:

Manim will look for a manim.cfg config file in the same directory as the file being rendered, and not in the directory of execution.

Running manim src/myanimation/animation1.py from a different directory than the scene file silently ignored src/myanimation/manim.cfg, and instead picked up (or missed) whatever manim.cfg happened to exist in the current working directory.

Fixes #4963

Fix

digest_args now re-digests library-wide + user-wide + the scene file's own directory's manim.cfg as one cascade once args.file is known (skipped when an explicit --config_file was passed, preserving existing precedence rules). This also fixes a secondary correctness issue: options the scene folder's config doesn't set now correctly fall back to the library defaults, instead of keeping whatever value a wrong cwd-based config had already set at import time.

Test plan

• Added test_folder_wide_config_resolves_relative_to_scene_file, which renders from a scene file in one directory while simulating stray config contamination, and asserts both that the scene folder's manim.cfg value is applied and that untouched options fall back to library defaults rather than leaking the contamination.
• pytest tests/test_config.py — 44 passed (6 unrelated failures in this sandbox are due to latex/LaTeX not being installed in the minimal test container, not this change)
• ruff check / ruff format --check clean

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