macOS: MoltenVK runtime, Apple Silicon builds and Studio verification #76

Current State

publish.yml builds osx-x64 on macos-latest, but the renderer is Vulkan-only and macOS has no native Vulkan
driver. Nothing enables the portability subset, packages MoltenVK, or builds for Apple Silicon (osx-arm64).

Tasks

• Enable VK_KHR_portability_enumeration / VK_KHR_portability_subset at instance and device creation
• Ship MoltenVK (and its ICD manifest) with the Studio, CLI and exported games
• Audit Vulkan features used against the portability subset
• Add osx-arm64 to the publish matrix; consider a universal bundle
• Verify window creation, HiDPI, input and the Studio end to end on Apple Silicon
• .app bundle, code signing and notarisation plan
• Update the platform docs

Acceptance Criteria

• Studio and an exported game run on an Apple Silicon Mac.
• Releases ship signed macOS artifacts for arm64 (and x64 if kept).

Adapted from the TurianZ (Zig) engine: turian#121