feat: add upsert and partial update APIs

[dcfocus]

Summary

• Add first-class append-only upsert by external_id in core, API trait, unified store, remote client, REST PUT /records, and Python sync/async APIs.
• Add partial update by id or external_id across core, API trait, unified store, remote client, REST PATCH /records, and Python sync/async APIs.
• Preserve identity, payload, embedding, and history for updates by writing a replacement record with supersedes_id while patching mutable metadata, relationships, scoping, and lifecycle fields.

Closes #81.

Notes

• The initial patch API sets provided fields and leaves omitted fields unchanged. Explicit null/clear semantics can be a follow-up if maintainers want that behavior.

Testing

• PASS: cargo-fmt --all --check
• PASS: cargo test --workspace
• PASS: cargo test -p lance-context --features remote
• PASS: cargo clippy --workspace --all-targets -- -D warnings
• PASS: uv run --project python --extra dev ruff format --check python/python/lance_context/api.py python/tests/test_search.py python/tests/test_persistence.py
• PASS: uv run --project python --extra dev ruff check python/python/lance_context/api.py python/tests/test_search.py python/tests/test_persistence.py
• PASS: uv run --project python --extra dev pyright python/python/lance_context/api.py python/tests/test_search.py
• PASS: uv run --project python --extra tests --reinstall-package lance-context pytest python/tests/test_search.py::test_context_update_requires_identifier_and_patch python/tests/test_search.py::test_context_update_returns_operation_metadata_and_record python/tests/test_search.py::test_context_update_missing_record_returns_not_updated -q
• PASS: uv run --project python --extra tests --reinstall-package lance-context pytest python/tests/test_persistence.py::test_update_by_external_id_patches_mutable_fields_and_preserves_payload -q
• NOTE: full-project pyright still reports pre-existing unrelated issues in examples/mcp-claude-code, python/tests/test_distance_metric.py, and older generator annotations in python/tests/test_persistence.py.