Fix backreference crash when optional group doesn't match

[emmanuel-ferdman]

PR Summary

Backreferences to optional capture groups that didn't participate in a match would crash with an ArraySliceError. For example, regex(r"(a)?\1") matching against "xaa" would fail because the engine tried to slice the input using uninitialized group indices. This adds a check to verify the referenced group actually matched before attempting to use it. If the group is unmatched, the backreference now correctly fails to match rather than crashing. The fix covers all matcher implementations: runtime backtracker, Thompson NFA, and compile-time regex.

Closes #10905

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dub run digger -- build "master + phobos#10933"