73aa6c89e2cb77c46184a81aec944e4ab99d114d  [Safer CPP] Mark more data members as `const` to help static analysis in WebCore

Classified as not security-relevant.
Triage note: Mass mechanical rename of setX() to initializeX() using lazyInitialize() and marking many unique_ptr/RefPtr members const across ElementRareData, HTMLMediaElement, Performance, etc. No bug identified; pure SaferCPP hardening refactor.
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 (+6.47), null_guard (+1.2), area:Source/WebCore/dom (+2.0), large_churn (-2.0)