Prepare 0.9: graphics interface trait redesign and module cleanup

[aosoft]

Summary

Final round of changes preparing the 0.9 release, plus the 0.8 → 0.9 migration guide. (Earlier 0.9-targeted refactors — unity-native-plugin-vulkan crate consolidation, sys-crate split, etc. — landed in previous PRs.)

Graphics interface traits

• Replaced inherent impl blocks on UnityGraphicsD3D11, UnityGraphicsD3D12* and UnityGraphicsVulkan* with traits mirroring the underlying IUnityGraphics* C interfaces; UnityGraphicsMetal* already used this pattern and is retained.
• D3D12 traits split into per-version (v2–v8) traits with macro-based implementations.
• Final trait suffix unified to *Interface across all back-ends (Metal's original convention). An interim rename to *Ext was reverted within the same branch.

Module cleanup

• Extracted ComPtr into a new windows module (was duplicated between d3d11 / d3d12).
• Inlined metal/objc2.rs into metal.rs.
• Inlined the define_unity_interface macro into lib.rs.

Features

• Merged profiler_callbacks into profiler.

Fixes

• Vulkan: VulkanInstance::get_instance_proc_addr now returns the standard ash Option<PFN_vkVoidFunction> instead of a sys-defined enum (unwrap_pfn removed).
• Vulkan interface error message typo: AccessTexture → AccessBuffer.

Docs

• Added the 0.8 → 0.9 migration guide to README.md covering all of the above plus changes from earlier 0.9 PRs (vulkan crate consolidation, ComPtr move, D3D11 typo fixes, GfxRenderer::ReservedCFE → Nvn2).
• Documented platform-specific feature gating (graphics features compile as no-ops on non-matching targets).

Test plan

• cargo build for each supported feature combination (d3d11, d3d12, vulkan, metal, profiler)
• cargo build on Windows, macOS, and a non-matching platform to confirm feature gating
• Verify unity-native-plugin-sample still builds against the new trait API