Preserve custom patterns during automatic backend selection

[harbinresearcher]

Summary

Closes #161.

PathSpec accepts custom Pattern implementations, but installing RE2 or Hyperscan causes the default backend to reject them. Select the existing simple backend when automatic selection encounters a non-RegexPattern. Explicit native selection, GitIgnoreSpec, and ordinary regex patterns retain their existing behavior.

Four regression methods exercise custom construction, callable factories, mixed include/exclude patterns and indices, negation, and + / +=. Constructor documentation describes the fallback.

Validation

• Pristine RED: four methods produced 11 subtest errors. Patched regressions pass.
• Linux CPython 3.12.15 with both native engines installed: baseline 266 tests OK, patched 270 tests OK, both with 1 skip.
• Windows CPython 3.12.14 with both native engines installed: baseline 266 tests OK, patched 270 tests OK, both with 16 skips. Windows 3.9 also reports 270 tests OK, with 645 skipped native/symlink subtests because optional engines were not installed there.
• Strict Sphinx HTML build, source distribution and wheel-from-sdist build passed. Installing that wheel and using custom patterns under all three backend preferences returns the expected matches.
• git diff --check passed. Mypy and Pyright retain exactly the same diagnostics as pristine master: 2 and 6 errors respectively. Type checking is not claimed as passing.

AI-assisted investigation, implementation, tests and read-only review with OpenAI Codex (GPT-6). No human maintainer review is claimed.

Remote CI: Test Cross run 38234033566 requires maintainer approval (action_required). No remote test pass is claimed.