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perf(regex): reuse the positions buffer across execute calls (#4267)

`execute` allocated a fresh positions array per call and checked its
length on every character. `Regex` now tracks `max_slot`, the largest slot
of any stabilized state (and of final-delta states), and keeps one scratch
buffer sized to it, so the per-character write is unchecked; the buffer is
re-checked only in the pending branch after a new state may have raised
`max_slot`. `execute` runs no caller code, so the buffer cannot be
re-entered, and every slot a result reads was written during the same call.

Native 1 MB scan 1.86 -> 1.55 ms, 20k-filename pattern 2.18 -> 1.88 ms,
34 short lines 4.97 -> 4.47 us; js 6.9 -> 5.7 ms. No interface change.
Part of #4263.

Codex CLI review: "Stabilization updates max_slot before the pending branch
calls ensure ... Each transition inherits initialized slots or assigns new
marks to its destination slot, which is written before checking for a
match. Allocation excludes surviving marks' slots, including nested
threads."

Signed-off-by: Codex CLI <codex@openai.com>
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

11 of 11 new or added lines in 4 files covered. (100.0%)

16535 of 18437 relevant lines covered (89.68%)

419367.77 hits per line