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

[frostming]

Please resolve conflict and add changelog

[gaoflow]

Done — rebased onto master (the conflict was just two adjacent new test functions; kept both) and added a changelog entry under Fixed. The bug still reproduces on current master: doc[k] = doc[k] normalizes [keys .'a'.'c'] to [keys.a.'c'], and this restores exact header preservation. Full suite green.