Add Metal capability probe for GitHub-hosted macOS runners

[wcandillon]

Summary

This PR adds a Metal capability probe to test whether Metal is usable on GitHub-hosted macOS runners, both on the host and inside the iOS simulator. This addresses the question of whether self-hosted Mac infrastructure is required for iOS end-to-end testing.

Key Changes

• metal-probe.swift: A comprehensive Metal diagnostic tool that:

  • Creates a default Metal device and logs device capabilities (name, family support, memory limits, etc.)
  • Executes a compute kernel that squares an array of floats and validates the results
  • Runs a render pass that draws a green triangle and verifies pixel output via CPU readback
  • Exits with code 0 only if both compute and render operations produce correct results

• metal-probe.yml: A GitHub Actions workflow that:

  • Runs on macos-latest to test the host environment
  • Collects system information (OS version, CPU, memory, Xcode version, display info)
  • Executes the Metal probe on the host
  • Boots an iOS simulator using the newest available iOS runtime and iPhone device type
  • Compiles and runs the Metal probe inside the simulator to test Metal availability there

Notable Implementation Details

• The Metal probe tests both compute and render pipelines to ensure comprehensive Metal functionality
• Device capabilities are logged to help diagnose differences across runner environments
• The iOS simulator test uses dynamic runtime/device selection to always test against the latest available versions
• The workflow is currently scoped to a specific branch for experimental validation

https://claude.ai/code/session_01WLhpP4ADhQTZnRtkdxd2Fg