xcodes/Xcode 26.x: bare `xcrun --show-sdk-version` can point at the CLT SDK; use `xcrun --sdk macosx` and download the Metal toolchain separately

After switching xcode-select to a freshly installed Xcode 26.3, xcodebuild -checkFirstLaunchStatus returned 0 and xcrun swiftc/git worked. But a bare xcrun --show-sdk-version still failed with 'SDK /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk cannot be located' (no DEVELOPER_DIR/SDKROOT set). xcrun --sdk macosx --show-sdk-path resolved correctly to Xcode's MacOSX26.2.sdk, so pass --sdk explicitly in scripts. Xcode 26 also no longer bundles the Metal compiler: xcrun metal errors 'missing Metal Toolchain' until you run xcodebuild -downloadComponent MetalToolchain. App builds that compile .metal shaders need that step after every new Xcode install.