219167e2b464d7d9ab4233bcaaa7c78d2035747c  [JSC] Shrink the DFG code that enters the OSR exit thunk

Classified as not security-relevant.
Triage note: Replaces linked-DFG patchable-jump OSR exits with nearCallThunk entrances indexed by return PC (osrExitReturnPC, osrExitIndexForReturnPC, m_osrExitEntrances) and drops m_patchableJumpLocation/codeStart encoding. JIT performance/structure refactor with asserts; no 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: security_assert (+4.87), null_guard (+1.2), area:Source/JavaScriptCore/dfg (+2.5)