[untoldengine]

Summary

• Reverts Warn at build time when Metal kernel sources are stale #1357 (commit 5cf2229), removing the MetalKernelsStaleCheck SwiftPM build-tool plugin and its stamp-writing hook in buildkernels.sh.
• In Xcode, the plugin triggers a malicious-code warning that must be manually trusted/approved, which is disruptive enough to warrant removing it.

Follow-up

We'll need another way to catch stale .metallib builds, since this was the problem #1357 was originally solving (closes #1351).

Closes #1384

Test plan

• git diff matches an exact revert of Package.swift / buildkernels.sh
• Confirm Xcode no longer prompts for plugin trust on a clean checkout