Preserve externally compiled regex flags during backend selection

[Master-Norna]

Both native backends rebuild RegexPattern.regex.pattern without its compile-time flags. As a result, installing an optional backend can silently change matching results for precompiled Python regexes.

This patch falls back to simple for None/best when an active standard-library re.Pattern has flags that cannot be reconstructed from its expression. Explicit re2/hyperscan selection raises ValueError with guidance to use backend='simple'. Comparing against the expression's own flags preserves existing inline-flag behavior; ordinary gitignore patterns keep their current native backend. This does not attempt to make all Python and native regex syntax or Unicode semantics equivalent.

It also makes += leave the spec unchanged if rebuilding the backend is rejected.

Regression tests cover I/M/S/X/A and combinations, Unicode case folding, verbose comments with invalid bare regex syntax, warning-as-error handling without leaking probe warnings, mixed inclusion/exclusion and indices, inline flags, no-op rules, GitIgnoreSpec directory precedence, and rejected in-place additions.

Validation on Windows / CPython 3.12.14:

• Full unittest discovery with both native libraries: 231 tests run, 14 existing Windows symlink skips, no failures/errors.
• tox -e ci-base,ci-re2,ci-hyperscan: all three isolated environments passed.
• Installed-wheel smoke tests in each tox environment: passed.
• Full discovery with -W error::FutureWarning, strict Sphinx build, and sdist/wheel build: passed.
• Mypy and Pyright each report the same two pre-existing _typing.py errors on the unmodified base and the patch, with no additional diagnostics.