#69 input: one compiled link regex per pattern for the whole process

Summary

• Each new surface compiled every configured link regex twice on the main thread: once for the surface (Surface.DerivedConfig.init) and once for its renderer (renderer/link.zig Set.fromConfig). In a cmux workspace switch trace (Time Profiler, 6 switches, cmux run 20261103T031956Z-ed80d47d) that was 6.3 ms of onig_compile per switch, about 0.5 ms per compile of the default URL regex.
• New src/input/link_regex.zig: one compiled regex for each pattern in the process, counted by its users. Surfaces and renderer link sets acquire on init and release on deinit. Up to 16 unused patterns stay compiled, so a switch that frees surfaces and then makes new ones compiles nothing. Matching does not change (same options as input.Link.oniRegex).
• Thread safety: Oniguruma 6.9.9 onig_search never writes to a regex_t (no reg-> assignment in regexec.c; the last search-time write, int_map_backward, was removed "for thread-safe", HISTORY 2016/07/17). Only compile and free change a regex_t, and both happen under the cache lock.
• Base: release/cmux-9ce9b7d0-e2 (cmux's pin 34e73ba8b), so cmux can pin it without the v1.4.0 / Zig 0.17 merge on main. A port to main is a follow-up.

Testing

• Red (test commit 5410e5d44, run 41574025483 on scratch branch scratch/tcc-link-regex-red, which only narrows the test filters): 2 of 2 new tests fail ("expected struct_re_pattern_buffer@...030, found ...230"); 82 other tests pass.
• Green (fix dfd07de4a, run 41574028017 on scratch/tcc-link-regex-green, which adds the new filters to the full patch filter list): ghostty-test 222/222, lib-vt 475/475, build, no-gettext and link smoke pass.
• The workflow's test filter list is not changed in this PR (no new CI test step). The scratch branches are never merged.

Changelog

none

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Summary by cubic

Shares one compiled link regex per configured pattern across all surfaces and renderers, so a new surface no longer compiles each regex twice on the main thread (about 0.5 ms per URL regex compile, 6.3 ms of onig_compile per workspace switch in cmux's trace).

• Adds src/input/link_regex.zig: a process-wide cache of compiled regexes, reference-counted by surfaces and renderer link sets (acquire on init, release on deinit).
• Patterns unused for a while stay compiled (up to 16), so a workspace switch that frees surfaces and creates new ones compiles nothing.
• Sharing is safe across threads: Oniguruma 6.9.9 search never writes to a regex_t; only compile and free mutate it, and both happen under the cache lock. Matching behavior is unchanged.
• Bases off release/cmux-9ce9b7d0-e2 so cmux can pin it without the v1.4.0 / Zig 0.17 merge; porting to main is a follow-up.