59be6c12449db1522f33431395e5dcb021c0a815  [JSC] Move the Collector implementation out of Heap.cpp

Classified as not security-relevant.
Triage note: Adds new heap/Collector.cpp with build-system wiring (CMake/xcodeproj/Sources.txt) moving existing collector-thread/phase logic verbatim; file reorganization refactor, no behavioral change.
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), clamp_saturate (+1.8), msg_noise (-2.5), large_churn (-2.0)