Harden Manim quality validation: off-screen layout checks and single-font lint

[jmjava]

Summary

This PR strengthens Manim quality safeguards with extended validation and tests focused on:

• off-screen / edge clipping and weak positioning
• readability and contrast clarity
• single-font-family consistency in scene source
• font pitfalls (weight=BOLD, positional Text color)

What changed

1) Extended layout/positioning checks (manim_layout.py)

LayoutValidator now performs broader frame sampling and stronger checks:

• configurable sampling interval across the full video
• edge safe-zone clipping detection
• overlap and minimum spacing checks
• text density checks (max_text_regions)
• minimum text-height readability checks (min_text_height_px)
• contrast ratio checks (min_contrast_ratio)

It now degrades gracefully when tesseract is unavailable:

• returns a passed report with a warning issue (tesseract unavailable; layout checks skipped)

2) Integrated Manim layout into main validator (validate.py)

Validator.validate_segment() now includes:

• layout_quality check for Manim segments

run_pre_push() treats layout_quality as a hard failure (not soft warning), so off-screen/layout regressions in Manim recordings are blocked.

3) Added static Manim scene lint (manim_scene_lint.py)

New source-level lint checks for animations/scenes.py:

• require Text.set_default(font=...)
• enforce expected single font family (manim.font, default Liberation Sans)
• detect mixed font families across defaults and explicit Text(..., font=...)
• deny weight=BOLD
• deny positional Text color arguments (e.g. Text("x", C_BLUE, ...))

Integrated into validate.py as manim_scene_lint for Manim segments.

4) Config and init updates

Config:

• added manim.font (default Liberation Sans)
• added validation.layout defaults for sampling/contrast/readability/density
• added validation.manim_lint defaults

init.py now scaffolds manim.font: Liberation Sans.

5) Demo scene cleanup to one font/no bold

Updated docs/demos/animations/scenes.py:

• set Text.set_default(font="Liberation Sans")
• replaced weight=BOLD usage with size/color emphasis
• removed explicit font="Monospace" overrides to keep a single family
• changed one check label text to avoid stale wording (no major layout issues)

6) Docs updates

README now documents new validation controls:

• manim.font
• validation.layout.*
• validation.manim_lint.*

docs/demos/docgen.yaml includes manim.font and example layout/lint config.

Tests

Added:

• tests/test_manim_layout.py (positioning/contrast/readability behavior)
• tests/test_manim_scene_lint.py (single-font and font pitfall linting)

Verification

• python3 -m pytest tests -q --ignore=tests/e2e -> 92 passed
• python3 -m ruff check src tests -> All checks passed

Issue linkage

Addresses #2, #9, #10 and enforceable source-level/font-safety parts of #4.