Problem: Some macOS environments report no devices via
MTLCopyAllDevices, causing startup failure with “unable to access a
compatible graphics device,” especially on Apple Silicon.
Change: Prefer MTLCreateSystemDefaultDevice
(metal::Device::system_default()) first. If None, enumerate devices and
select a non‑removable, low‑power device by preference.
Why this works: On Apple Silicon the system default is the unified GPU;
on Intel, the fallback keeps a stable policy and avoids accidentally
picking removable/high‑power devices.
Impact: Fixes startup on affected ASi systems; improves selection
consistency on Intel multi‑GPU. Behavior unchanged where
system_default() succeeds.
Risk: Low. Aligns with Apple’s recommended selection path. Still fails
early with a clearer message if no Metal devices exist.

Closes #37689.

Release Notes:
- Fixed: Startup failure on some Apple Silicon machines when Metal
device enumeration returned no devices by falling back to the system
default device.

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Co-authored-by: 张小白 <364772080@qq.com>
Co-authored-by: Kate <work@localcc.cc>