A regular expression search is bounded by a step budget patched into mvzr, not windows and a repeat cap

The windows returned wrong matches: a candidate reaching a window's edge was
left to the next window, half a window on, which could answer a match starting
mid-token rather than the leftmost, and addr then pointed data's next write at
the wrong bytes. The repeat cap missed mvzr's own worst case, a chain of a?,
and alternation under a repeat, each exponential inside one mvzr call the
deadline could not interrupt; and it refused ordinary s/S patterns. build.zig
now patches the fetched mvzr at build time with a step counter on its
backtracking recursion (matchPattern), so a fresh fetch keeps it and a moved
anchor stops the build; regexp.zig gives each compiled pattern a budget, about
300 ms here, and a search that spends it fails as taking too long. Windows,
the cap and their special cases are gone, and matches are exact again.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

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