Preserve table display names on self assignment

[gaoflow]

Fixes #291.

Summary

• avoid invalidating parsed table display names when a table is assigned back to its existing slot
• add a regression test for preserving whitespace and quoting in dotted table headers after self-assignment

Verification

• uv run --no-project --with pytest --with pyyaml python -m pytest tests/test_toml_document.py::test_replace_table_with_itself_preserves_display_name tests/test_toml_document.py::test_replace_with_table_of_nested tests/test_toml_document.py::test_nested_table_update_display_name -q
• uv run --no-project --with pytest --with pyyaml python -m pytest -q
• uv run --no-project --with ruff ruff check tomlkit/container.py tests/test_toml_document.py
• uv run --no-project --with ruff ruff format --check tomlkit/container.py tests/test_toml_document.py
• python -m compileall tomlkit/container.py tests/test_toml_document.py
• git diff --check