fix: isolate aliased configuration mappings

[codeforester]

Fixes #407. Copies nested configuration containers so shared YAML aliases cannot make unrelated effective values mutate together. Tests: tests/test_batteries_included_config.py.

[codeforester]

Review: the fix is correct (_copy_config_value deep-copies mappings, lists and tuples, so aliases are no longer shared). One test problem to fix before merge.

The new test_nested_mapping_aliases_are_isolated_across_layers was inserted in the middle of test_shared_mapping_alias_is_valid_but_recursive_alias_is_rejected_with_path. The recursive-alias with tempfile.TemporaryDirectory() block that belonged to the original test now sits at the end of the new test. Nothing fails, but the original test no longer checks what its name says, and the new test has an unrelated recursive-YAML assertion. Please move that block back to the original test.

Non-blocking: a tuple branch in _copy_config_value is untested (YAML won't produce tuples, so it only matters for programmatic callers).