fix: resolve PUBLIC before looking up the user home

[emme1t]

Windows().user_publicshare_dir raises RuntimeError: Could not determine home directory when PUBLIC is set but the home-directory environment variables are unavailable. The default argument to os.environ.get() evaluates Path("~").expanduser() before the environment lookup returns.

Resolve the home-based fallback only when PUBLIC is absent. Existing normalization, fallback paths, and empty-string behavior are preserved. Add regressions for an unavailable home with an explicit public path and for the fallback when PUBLIC is absent, plus a news fragment.

Reproduced on Windows with Python 3.13:

import os
from unittest.mock import patch
from platformdirs.windows import Windows

with patch.dict(os.environ, {"PUBLIC": r"C:\Users\Shared"}, clear=True):
    print(Windows().user_publicshare_dir)

Before the fix this raises RuntimeError; afterward it returns C:\Users\Shared.

Validation:

• New regression on original source: the explicit-public-path case fails and the fallback case passes.
• tox r: Python 3.12 and 3.13 each pass 1,243 tests with 94 skips. Other configured interpreters are unavailable locally and skipped by tox.
• Coverage, documentation, package metadata/build checks, and type checks pass. All changed executable lines have 100% diff coverage; combined local coverage is 94.50%.
• The full local formatting environment reports pre-existing YAML comment-spacing and changelog-formatting differences. Running the configured pre-commit hooks on the two changed Python files and the news fragment passes. The repository's pre-commit.ci bot subsequently applied the changelog formatting in its own commit, which is retained in this PR.
• git diff --check passes.

Prepared with OpenAI Codex assistance. The reproduction and checks above were executed locally, and the patch received an independent agent review.