Avoid capturing dotted keys after empty table replacement

[gaoflow]

Fixes #513.

Summary

• track whether rendering is still in the root/current table scope before emitting dotted assignments
• force a table header only when a super table would otherwise emit dotted items in the wrong scope
• add root and nested regression coverage for replacing a dotted-key table with an empty table

Verification

• uv run --no-project --with pytest --with pyyaml python -m pytest tests/test_toml_document.py::test_replace_dotted_key_table_does_not_capture_following_dotted_key tests/test_toml_document.py::test_replace_nested_dotted_key_table_keeps_following_dotted_key -q
• uv run --no-project --with pytest --with pyyaml python -m pytest tests/test_toml_document.py -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