fuzz-macos: mark as continue-on-error (best-effort)

Homebrew LLVM 18 runtime dylibs use typed allocation ABI symbols
(__ZnwmSt19__type_descriptor_t) not available in macOS 14's system
libc++, causing dyld to abort. Xcode clang doesn't ship libFuzzer.

Mark fuzz-macos as continue-on-error (same as fuzz-windows) so it
doesn't block CI. Linux fuzzing remains the primary bug detector.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>