feat(corpus): add resumable captioned corpus-v2 builder

[vex0209-bt]

Summary

• add a deterministic, resumable corpus-v2 builder for five-second TI2V truth clips and lossless frame-zero PNGs
• caption prepared clips with a pinned Qwen3-VL revision using one persistent worker per selected GPU
• enforce motion-stratified human QA before verified, content-addressed Hippius publication
• generate digest-anchored manifest shards and publish the immutable root last
• document the offline build, review, publish, and staged scaling workflow

Safety boundaries

• leaves chain.toml and the active corpus-v1 evaluator unchanged
• refuses partial manifest roots and verifies uploaded artifact size plus SHA-256 metadata
• requires evaluator-v2 support and calibration before a corpus-v2 root may be pinned into consensus
• preserves restart safety through a SQLite ledger with atomic caption claims and publication checkpoints

Verification

• PYTHON_DOTENV_DISABLED=1 LEOMA_EVAL_TOKEN= OBJECT_STORAGE_BACKEND=r2 venv/bin/pytest -q: 646 passed
• new corpus-v2 and CLI tests: 24 passed
• uv lock --check: passed
• uv build --wheel: passed
• git diff --check: passed
• pyflakes reports only five pre-existing warnings outside this change