101392f63ca9966babdf9757a1a85793f2b38073  [JSC] Remove SlotVisitor's Heap in preparation for a visitor that marks more than one heap

Classified as not security-relevant.
Triage note: Removes AbstractSlotVisitor::m_heap in favor of m_collector.heap() and captures CollectionScope/HeapAnalyzer at didStartMarking (also drops heap/SlotVisitor.h from ForwardDeclCheckerExpectations); a SaferCPP-adjacent GC refactor with no identified exploitable 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 (+3.0), security_assert (+3.0), msg_soft (+1.0), area:Source/JavaScriptCore/runtime (+2.5), large_churn (-2.0)