feat: add logical delete/forget API

[dcfocus]

Summary

• Add Context.delete()/forget() and PyO3 bindings for id or external_id deletes
• Store deletes as internal tombstone records so default list/get/search hide forgotten rows
• Add Rust/Python tests and docs for logical delete semantics

Testing

• uv run --with maturin maturin develop
• uv run --with pytest pytest python/tests/ -v
• uv run --with pytest pytest tests/test_delete.py tests/test_search.py -v
• uv run --with ruff ruff format --check python/
• uv run --with ruff ruff check python/
• uv run --with pyright pyright
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test -p lance-context-core --lib
• cargo test -p lance-context-core --doc

Note: broader local python/tests run has 4 pre-existing raw-fragment/compaction failures with MemWAL visibility (test_compaction..., test_persistence...). Focused delete tests and the exact workflow test path pass.

Closes #55