956f6fb49678a6f6b0116325be5b5357df948970  [YARR] Match lookbehinds in the interpreter from a copy of the disjunction in match order

Classified as not security-relevant.
Triage note: Large systematic refactor of the YarrInterpreter input cursor (readChecked/readCheckedDontAdvance/checkInput/positionAt) to carry MatchDirection for forward/backward matching. No single identified bug; RELEASE_ASSERT bounds are preserved/reorganized, consistent with plumbing rather than a fix.
This commit was flagged by the signal scorer and reviewed by AI (not a human). It is listed only so the triage is auditable; there is no root-cause analysis because this is not a vulnerability fix.

Structural signals: security_assert (+6.47), bounds_guard (+2.0), null_guard (+2.59), large_churn (-2.0)