Parse nested config sections independently of parent declaration order

[sueun-dev]

A child section before its declared parent currently fails during config loading. For example, Config().from_str("[a.b]\nx = 1\n[a]") raises KeyError: 'a' on b3adaf4, while the same sections in parent-first order load successfully.

Build the section skeleton in increasing depth before filling its values. The regression test covers all 24 orderings of two parent/child pairs: 18 fail on the original source, and all 24 pass with the change. Missing-parent and conflicting-value rejection remain covered by the existing tests.

Validated locally on Python 3.12.14 / Linux aarch64:

• python -m pytest tests/ -q: 305 passed, including against the installed sdist with the source package moved out of the test directory.
• Ruff lint/format, Pyright, and source/wheel builds passed.
• A separate probe passed 720 section permutations through string and byte loaders, plus disk loading, rejection controls, and positional sibling-order preservation.

Other Python versions and operating systems were not tested. Prepared with AI assistance; these checks were executed locally.