fix(cli): resolve output directories before invoking backends

[harbinresearcher]

Summary

Resolve the CLI's selected output directory against the caller's working directory before passing it to the frontend. Previously, python -m pyproject_api project --wheel created project/dist in the caller's directory, then sent project/dist to a backend already running inside project, causing the demo backend to raise FileNotFoundError.

The runtime change is one expression. Add nine end-to-end cases using the repository's existing demo backend for default, relative and absolute output paths across sdist/wheel/editable builds. They check real generated artifacts; no subprocess behavior is mocked. The pristine source fails six cases, while the three absolute-path controls pass. Adjust two existing argument-test expectations and document the CLI path interpretation.

This keeps symlink interpretation intact (absolute(), not resolve()) and leaves generic frontend path/configuration arguments unchanged.

Validation

• Linux Python3.11.17,3.12.15,3.13.16,3.14.8, stable3.15.0 and free-threaded3.15.0: official tox baseline74 passed → patched83 passed in every environment, all reaching the configured100% coverage gate.
• Official tox fix, type, docs and pkg_meta passed; fixed pre-commit hooks did not modify the three candidate files. Linux tested copies match the canonical worktree byte for byte.
• Windows Python3.12.14: CLI tests16 passed; full-suite baseline73 passed/1 failed and patch82 passed/1 failed. The same unchanged test_ensure_empty_dir_on_path_symlink_to_folder fails with WinError1314 because this account cannot create directory symlinks.
• The original CLI repro now creates the real demo wheel at the expected path.
• Read-only AI review found no actionable issue; no independent human review is claimed.

Hosted CI and macOS are unverified. AI assistance was used for investigation, tests, implementation and review; no production incident is claimed.

Closes #315

Remote verification: ReadTheDocs and pre-commit.ci succeeded. Actions run38507824873 is action_required, awaiting maintainer approval to run; hosted tests are not yet verified.

[gaborbernat]