Resume std::regex searches on libc++ without its ^ at the range start, and name the legacy-anchor cases libc++ cannot parse in the native unit

[puskuruk]

Two regex failures on macOS (Apple clang 21, libc++ 210106) that exist on main: test/runtime/regexp-anchors-line-terminators.ts and the native unit regexp-legacy-anchors-runtime. CI runs on Ubuntu, where neither shows.

^ at a resumed search. libc++'s basic_regex::__search clears match_not_bol whenever match_prev_avail is set, and its ^ accepts the first position of any search range outright, so a search resumed at lastIndex matched ^ there after any code unit: 'aa\na'.match(/^a/g) found two matches, 'a\nb'.replace(/^|$/g, '<>') gave <>a<>\n<>b<>, a sticky /^a/y at 2 matched in x\na. libstdc++ honours the flag. The runtime now resumes through Pattern::searchFrom: on libc++ a pattern with a ^ outside a class is searched in a caret-free form (each such ^ replaced by [^\s\S]) wherever no ^ can hold, and in its own form only from a position where one can -- position 0, or after a line terminator with m. Every other library takes the plain call it took before. The three resumed searches (exec, split, and the legacy path's unrewritten form) go through it.

The native unit. The rewrite a library without multiline needs checks a ^ reached after consuming with (?=\1). libc++ parses a lookahead as a pattern of its own, so a back reference to an outer group inside one is rejected, and the unit, which simulates that library by compiling every pattern multiline, aborted on a\n^b. libc++ has multiline and never takes the rewrite outside the simulation; the unit now names the cases that need that form and leaves them out on libc++ (14 of 87), running the rest.

Verified on this Mac: scripts/check-runtime-header.mjs passes; all 18 regexp* runtime programs pass including the one above; the native unit passes 73 of 87 with 14 named; npm run gate reports 0 drift. Verified in the Linux (libstdc++) container: the same 18 programs pass, and the unit passes all 87 cases (built without sanitizers there, the container has no ASan runtime for clang 18).

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Reviewing files that changed from the base of the PR and between f9154e8 and eac5323.

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• src/targets/cpp/runtime/gea_runtime.h
• test/runtime/regexp-legacy-anchors-runtime.cpp

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[puskuruk]

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[puskuruk]

Superseded by c0abb2d ("Resume regex searches correctly on libc++", released in 1.0.40), which fixes the same libc++ ^ resume at lastIndex through the search flags. On 1.0.41 check-runtime-header passes on macOS without this branch, so closing.