95a960a3ae94afa466bba4932de879236a178c12  Reduce use of CString::legacyCStringPointer() in assertion and signpost macros

Classified as not security-relevant.
Triage note: Mechanically drops .legacyCStringPointer() calls, passing CString/String directly to FINALIZE_CODE/signpost/RELEASE_ASSERT_WITH_MESSAGE and switching nullptr handling to isNull(); confined to JSC JIT/GC disassembly, signpost and assertion-message paths, a logging refactor with no execution-path 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 (+6.47), msg_noise (-2.5), area:Source/JavaScriptCore/bytecode (+2.5), large_churn (-2.0)