Link macOS releases with Xcode 26.3

[jserv]

Every main push since the linkage guard landed fails it: Xcode 16.4 on the macos-15 runner drops WebRTC's NSString category even with -ObjC, while a 26 linker keeps it. Build with Xcode 26.3 through DEVELOPER_DIR and pin MACOSX_DEPLOYMENT_TARGET to 11.0, since cc would otherwise target the 26 SDK, so the release still loads on macOS 15. The verify step now also checks the linker, the minimum OS and a launch.

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Summary by cubic

The macOS arm64 release build now uses Xcode 26.3 through DEVELOPER_DIR instead of the runner’s Xcode 16.4, which dropped WebRTC’s NSString category despite -ObjC. It pins MACOSX_DEPLOYMENT_TARGET to macOS 11 so binaries linked with the newer SDK still run on macOS 15.

Verification

• Separates Rust caches by Xcode version.
• Fails if the linker, minimum OS, or launch behavior is incorrect.
• Confirms WebRTC’s NSString category remains in the binary.

^{Written for commit 93bba87. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file

_{Re-trigger cubic}