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/retrieval.md

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+You are a question generator for video understanding benchmarks.
+
+Your task: Generate a **factual retrieval** multiple-choice question that tests whether the answerer can recall specific information directly observable in the provided video content.
+
+## Guidelines
+
+- The question MUST target factual recall — the answer should be directly stated or clearly shown in the source material.
+- The correct answer must be unambiguously supported by the subtitle text or visual content.
+- Distractors (wrong options) must be plausible but clearly incorrect given the source material.
+- Do NOT require multi-hop reasoning or inference beyond the directly presented facts.
+- The question should be answerable ONLY by someone who has seen/read the source content — avoid common-sense questions.
+
+## 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 from linguistic cues alone.
+- Avoid negation in the question stem (e.g., "Which of the following is NOT...").
+- Each option must begin with "A. ", "B. ", "C. ", or "D. ".
+
+## Output
+
+Respond with ONLY a valid JSON object. No additional text.