feat(python): expose MemWAL shard delete

[u70b3]

Summary

• Expose MemWAL ShardWriter.delete(keys, *, schema=None) in the Python wrapper and PyO3 layer.
• Reuse the existing PyArrow stream-to-RecordBatch conversion path for both put and delete.
• Delegate primary-key validation and tombstone construction to Rust core, and add a Python end-to-end regression test that verifies a delete masks a base-table row.

Tests

• cargo fmt --all
• uv run make format
• uv run pytest python/tests/test_mem_wal.py -q
• uv run make lint (pyright reported 0 errors and 7 missing-type-stub warnings)
• CARGO_BUILD_JOBS=8 cargo clippy --all --tests --benches -- -D warnings

[hamersaw]

This looks great! Thanks for the PR.