fix(ci): build darwin binaries on macOS runners to restore code signing

v3.16.0 binaries had 'adhoc,linker-signed' signatures because newer
Bun versions no longer emit linker-signed signatures when cross-compiling
darwin targets from Linux. Current releases have 'code object is not
signed at all', causing macOS Gatekeeper to reject them immediately.

Verified empirically:
- v3.16.0 darwin-arm64: Signature=adhoc, flags=0x20002(adhoc,linker-signed)
- v3.17.0 darwin-arm64: 'code object is not signed at all'
- Local bun build on macos: produces linker-signed signature (matches v3.16.0)

Changes:
- Route darwin-* platforms to macos-latest runner (native compile)
- Add explicit ad-hoc codesign step as belt-and-suspenders safety net