Fix pipeline correctness: quality mapping, stale VHS warnings, ffmpeg timeout, binary paths

[jmjava]

Summary

Implements a focused pipeline-correctness pass covering:

• Manim quality mapping: 1080p30 falls back to 720p30 silently #6 Manim quality mapping and source resolution
• docgen manim/vhs not found in PATH when run via venv entry point #7 Manim/VHS binary path resolution
• Docs/UX: clarify compose vs vhs — stale rendered/*.mp4 causes freeze guard failures #12 stale VHS source warning in compose
• Feature: configurable ffmpeg timeout for Manim/compose (avoid false ERROR on long scenes) #13 configurable ffmpeg timeout

What changed

1) Manim quality mapping + path resolution (#6)

• Added robust quality parsing in ManimRunner:
  • Presets: 480p15, 720p30, 1080p60, 2160p60
  • Parsed custom forms like 1080p30, 1440p30, etc. via explicit --resolution + --frame_rate
• Added clearer render logging that prints the effective quality configuration.
• Updated compose Manim source lookup to search quality directories based on configured manim.quality (with fallbacks), including both:
  • animations/media/videos/scenes/<quality>/...
  • animations/media/videos/<quality>/...

2) Binary resolution for manim/vhs (#7)

• Added src/docgen/binaries.py with shared binary resolution helper.
• manim resolution now supports:
  • manim.manim_path from config
  • venv-local executable resolution (next to sys.executable)
  • PATH fallback
• vhs resolution now supports:
  • vhs.vhs_path from config
  • common install locations (~/go/bin/vhs, /usr/local/bin/vhs, /snap/bin/vhs)
  • PATH fallback
• Improved actionable error output with paths attempted and config hints.

3) Stale VHS warning in compose (#12)

• Added stale source warning before composing VHS segments when enabled:
  • warns if terminal/*.tape is newer than terminal/rendered/*.mp4
• Configurable via compose.warn_stale_vhs (default true).

4) Configurable ffmpeg timeout (#13)

• Added config option compose.ffmpeg_timeout_sec (default 300).
• Added CLI override: docgen compose --ffmpeg-timeout <seconds>.
• Updated ffmpeg handling:
  • raises ComposeError for hard failures (missing ffmpeg, non-zero exit, timeout without output)
  • emits a warning when timeout occurs but an output file already exists.

5) Config/docs/init updates

• Added new config accessors in Config:
  • manim_path, vhs_path, ffmpeg_timeout_sec, warn_stale_vhs
• Updated docgen init generated config to include manim_path, vhs_path, and compose timeout/stale warning options.
• Updated README with:
  • new compose CLI option
  • config snippet for quality/path/timeout/stale warning
  • explicit vhs -> compose guidance after tape edits

Tests

• Added new tests:
  • tests/test_manim_runner.py
  • tests/test_compose.py
• Extended config tests in tests/test_config.py.
• Full unit test run:
  • python3 -m pytest tests -q --ignore=tests/e2e -> 81 passed
• Lint:
  • python3 -m ruff check src tests -> All checks passed

Issue linkage

Closes #6
Closes #7
Closes #12
Closes #13