feat(datagen): add single-log checkpoint store

[beinan]

Summary

• add a first-class DatagenStore backed by one append-only Lance log, avoiding cross-dataset consistency hazards
• model item lifecycle, field deltas, failures, terminals, no-op step completion, deterministic retry ids, fold, and trajectory reads
• use multi-shard MemWAL ingest with a resident writer, retry deduplication, per-shard cleanup, and row-id blob point reads
• document the single-log schema and re-export the public API from lance-context

Testing

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test -p lance-context-core datagen --no-fail-fast (8 passed)
• cargo test -p lance-context-core --no-fail-fast: 148 unit tests passed; the existing rollout wal_merge_generation_cleanup integration test fails on current main after perf(rollout): reuse a resident MemWAL ShardWriter across appends #159 with 32 rows observed for 30 appends
• uv run ruff format --check python/
• uv run ruff check python/
• uv run pyright
• uv run pytest: 181 passed, 5 skipped, 1 xfailed; 10 unrelated current-main failures in stale DummyInner signatures and moto S3 bucket visibility