[liuyinchu]

A precompiled google-re2 expression with Options.case_sensitive = False matches SECRET with the simple backend, but selecting an accelerated backend recompiles only its pattern text and loses that option. The default best selection can therefore change inclusion/exclusion results merely when an optional backend is installed.

Return unsupported from the existing foreign-expression guard, as its comment already intends. Automatic selection then retains the original compiled object with the simple backend; explicit native selection returns the existing guidance to use backend='simple'. Native Python patterns retain their existing path, and inactive foreign patterns do not force fallback.

Adds proxy and real-RE2 regressions, inactive-pattern/index controls, and synchronized documentation. Ten failing baseline subcases become passing; the full Python 3.12 suite runs 298 tests successfully with one existing skip and both native backends installed. Another 84 option/order/exclusion cases pass in independent review. Mypy and Pyright findings are byte-identical on baseline and patch; other Python/OS combinations were not run.

This covers foreign compiled objects, distinct from the stdlib flag preservation in #151.

AI-assisted investigation, implementation, tests and writing; independently reviewed. No production incident is claimed.