c2fa19276047500b65b8a582c68b0e6b1c928974  [JSC] Split the GC's collection-cycle state and machinery into a Collector class

Classified as not security-relevant.
Triage note: Introduces heap/Collector.h and threads a Collector& through AbstractSlotVisitor constructors, with xcodeproj/CMake header wiring and SaferCPPExpectations updates. GC refactor/rename; no security bug addressed.
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), security_assert (+6.47), null_guard (+2.59), msg_noise (-2.5), large_churn (-2.0)