macOS gold via MoltenVK (real Apple GPU path) + some CompareGold improvements

[xen2]

PR Details

macOS gold via MoltenVK (real Apple GPU path)

Switches macOS test rendering from Lavapipe (CPU) to bundled MoltenVK, Apple's actual Vulkan-on-Metal path.
Captured new golds under per-GPU-family buckets Apple M4.
MoltenVK loader is pinned via VK_DRIVER_FILES from a ModuleInitializer, so brew's molten-vk is no longer needed (temporary until KhronosGroup/MoltenVK#2731 is fixed)

Lavapipe stays available on macOS via STRIDE_TESTS_GPU=0.

CompareGold tool changes

• Per-run metadata sidecar (*.metadata.json) — captures OS / CPU / GPU / driver name + version / Vulkan API version, sourced from VkPhysicalDeviceDriverProperties (Vulkan) and IDXGIAdapter (D3D). Main sidecar renamed .json → .results.json so the two never collide.
• Driver-derived bucket names — Apple M4, Lavapipe, WARP, etc., derived from driver identity instead of the previous SW_RENDERING env hack. On Apple paravirtualized GPUs (macOS VMs / GH Actions), falls back to the CPU brand string for a stable family bucket.
• CompareGold: metadata pill under each image with hover-popup full table + reveal-in-explorer; real CI run_number (#22) in source labels; PNG-less platforms greyed out; auto-select first source-bearing platform on add/remove; --port / --lan flags.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My change requires a change to the documentation.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have built and run the editor to try this change out.