09104a6e1e5dd370ef0b0a85eae08b6d0e7920c4  [Site Isolation] matchIndex should track position across remote frames, not just per-frame.

Classified as not security-relevant.
Triage note: Stores per-frame matchCount in FindStringCallbackAggregator and computes globalIndexOffsetForFrame plus FindOptions::NoIndexChange; corrects the reported match index under site isolation, not a security bug.
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: smartptr_member_added (+4.87), surgical (+1.3)