41c908b6b6717d5dc5a4c82d87ec1e252e9cfe87  [Site Isolation] Avoid re-broadcasting frame geometry if nothing changed

Classified as not security-relevant.
Triage note: Adds operator== to FrameGeometrySyncData/RemoteFrameLayoutInfo and caches m_lastBroadcastFrameGeometry to skip redundant broadcasts, with rebroadcast on frame navigation. Cross-process sync optimization/correctness, no memory-safety or security-logic 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), null_guard (+1.2)