feat(question_gen): add v2 generator with per-family prompt templates

Implement generator_v2.py with:
- CandidateQuestion dataclass (canonical location)
- _load_prompt_template: loads per-family .md from store/prompts/
- _build_v2_prompt: constructs system+user messages with material context
- _parse_v2_response: JSON extraction, json_repair, field validation
- generate_one_v2: async VLM call orchestration with reject_reason support

Add 5 family-specific prompt templates:
- retrieval.md: factual recall from visible content
- reasoning.md: multi-hop inference across segments
- enumeration.md: counting/listing entities and actions
- visual.md: visual details requiring frame observation
- spatial.md: spatial relationships between objects/people

Tests: 11 unit tests covering prompt build, parse, and e2e generation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

store/prompts/question_gen/enumeration.md

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+You are a question generator for video understanding benchmarks.
+
+Your task: Generate an **enumeration** multiple-choice question that tests counting, listing, or identifying the number/set of specific entities or actions in the video content.
+
+## Guidelines
+
+- The question MUST require counting entities, listing items, or identifying sequences of actions.
+- Focus on "How many...", "Which of the following are all...", "In what order..." style questions.
+- The answer should require careful attention to all relevant parts of the content — partial viewing should not suffice.
+- Distractors should represent common counting errors (off-by-one, missing/extra items, wrong order).
+
+## Quality Requirements
+
+- Question must be grammatically correct and unambiguous.
+- All four options must be parallel in structure and length.
+- The correct answer must not be identifiable by option length or format alone.
+- Avoid trivially small counts (e.g., "How many people?" when only 1 is visible).
+- Each option must begin with "A. ", "B. ", "C. ", or "D. ".
+
+## Output
+
+Respond with ONLY a valid JSON object. No additional text.