feat: expand explainer authoring with attention, Manim, and capture QA

[makesynt]

Summary

This PR expands Seqvio from a basic composition renderer into a deterministic
technical explanation pipeline.

Explainer graphics and attention guidance

• Add InfographicScene for metrics, comparisons, processes, timelines,
  relationships, and charts.
• Extend shared annotation primitives with spotlight, focus ring, callout,
  bracket, connector, region shade, and guided path.
• Add deterministic collision-aware placement and safe connector routing.
• Add random-access, reverse-seek, overlap, focal-target, and minimum-hold QA.
• Add semantic DirectionPlan and Motion Grammar contracts.
• Add host-agent director tasks for generating and repairing direction artifacts.
• Add versioned style profiles:
  • clean-technical
  • editorial-explainer
  • terminal-first

Python Manim integration

• Add experimental @seqvio/manim-adapter.
• Invoke the external Python package manim through python -m manim.
• Add deterministic scene validation and command construction.
• Add Python/Manim preflight checks and configurable Python executable support.
• Add content-addressed render caching based on source, assets, settings, and
  runtime versions.
• Add media probing, progress events, cancellation, timeout handling, logs,
  diagnostics, and machine-readable render manifests.
• Add ManimClip for deterministic playback of externally rendered media.
• Add ExplainerDocument support for manim scenes.
• Align named Manim markers with narration phrases and ExplanationBeats.
• Add equation, graph, symbolic-proof, and geometric-proof fixtures.

Capture and release pipeline

• Add deterministic browser privacy masks using explicit selectors or regions.
• Promote Terminal and Browser capture contracts to stable adapter compatibility.
• Improve capture lifecycle, artifact manifests, QA reports, and progress output.
• Stabilize Windows capture smoke timing and cross-platform CI verification.
• Add render benchmarks and release contract validation.
• Synchronize the lockfile and package lifecycle metadata.

Documentation

• Synchronize English and Chinese README files.
• Add the complete Python Manim integration guide.
• Add package-level documentation for @seqvio/manim-adapter.
• Update the roadmap, lifecycle policy, capability reference, troubleshooting
  guide, examples, and public feature-status documentation.
• Remove obsolete planning documents and align current status wording.

Design boundaries

• @seqvio/manim-adapter is a TypeScript/Node.js adapter, not a JavaScript
  implementation of Manim.
• Python Manim is optional and only required when generating mathematical
  animation.
• Manim rendering happens externally; ManimClip consumes the resulting
  seekable media inside Seqvio.
• Seqvio does not call AI or planner APIs. Host agents produce the semantic
  plans, while Seqvio validates and executes them deterministically.
• Terminal and Browser capture preserve real observed evidence and timing.

Validation

• npm run build
• npm test
• npm run verify:contracts
• npm test --workspace @seqvio/manim-adapter
• Cross-platform release smoke verification on Windows, Linux, and macOS
• Manim media probing, cache reuse, marker alignment, and seek QA
• Relative Markdown link validation and git diff --check

Review focus

Please pay particular attention to:

1. The boundary between external Python Manim rendering and Seqvio playback.
2. ExplanationBeat and Manim marker reflow after TTS resolution.
3. Deterministic attention routing and reverse-seek behavior.
4. Browser privacy-mask semantics and their security limitations.
5. Lifecycle and release-policy changes for experimental packages.