feat(question_gen): add v2 material sampler with family constraints
Implement sample_material_v2 module that samples tree nodes with
QuestionFamilySpec-aware constraint validation, providing richer
MaterialContext output (subtitles, cross-L2 context, frame paths).
Key components:
- AnchorContext/MaterialContext frozen dataclasses
- _validate_sampling_constraints: multi-level constraint checking
- _collect_subtitle_sentences: subtree subtitle extraction
- _collect_cross_l2_context: peer L2 event descriptions
- sample_material_v2: main entry with retry-on-constraint-violation
Tests: 11 unit tests covering normal sampling, used-node exclusion,
constraint violation retries, cross-L2 population, and subtitle
collection.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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