Unify backend dispatch, add thread safety, depth testing, and API docs

- Add missing Metal dispatch for begin/end frame, destroy_texture,
  shader/pipeline lifecycle, and resize in sc_gfx.h
- Wire D3D12 and WebGPU stubs into sc_gfx.h dispatch for all 14
  backend-routed functions with CMake backend options
- Add sc_metal_resize implementation, sc_d3d12_resize/sc_wgpu_resize stubs
- Implement depth/stencil pipeline state, depth buffer, depth testing
  and depth write in software rasterizer
- Add sc_gfx_lock/unlock with platform mutex (POSIX/Windows/WASM)
  and thread_safe option in SCGfxDesc
- Add Doxygen API docs on all 19 public functions
- Add depth, thread-safety, and resize test cases (26 total)
- Update Python bindings for resize, lock, unlock
- Add Vulkan pipeline cache with desc hashing
- Update sc_kernel.c unity build for D3D12/WGPU backends

Author: amalxloop

CMakeLists.txt

@@ -11,8 +11,8 @@ option(SC_TESTS    "Build test suite"                         ON)
 option(SC_SHARED   "Build shared library (libsc_kernel.so)"   ON)
 
 # Graphics backend selection
-set(SC_GFX_BACKEND "SOFTWARE" CACHE STRING "Graphics backend: SOFTWARE, VULKAN")
-set_property(CACHE SC_GFX_BACKEND PROPERTY STRINGS SOFTWARE VULKAN)
+set(SC_GFX_BACKEND "SOFTWARE" CACHE STRING "Graphics backend: SOFTWARE, VULKAN, METAL, D3D12, WGPU")
+set_property(CACHE SC_GFX_BACKEND PROPERTY STRINGS SOFTWARE VULKAN METAL D3D12 WGPU)
 
 # ---------------------------------------------------------------------------
 # Global compile settings
@@ -52,6 +52,21 @@ if(SC_GFX_BACKEND STREQUAL "VULKAN")
     if(NOT XCB_FOUND AND NOT WAYLAND_FOUND)
         message(WARNING "No window system found; windowed Vulkan mode disabled, headless only")
     endif()
+elseif(SC_GFX_BACKEND STREQUAL "METAL")
+    set(SC_GFX_BACKEND_DEF "SC_GFX_BACKEND_METAL")
+    set(SC_GFX_LIBS "")
+    set(SC_GFX_INCLUDES "")
+    message(STATUS "Metal backend selected (compile .mm files with -fobjc-arc)")
+elseif(SC_GFX_BACKEND STREQUAL "D3D12")
+    set(SC_GFX_BACKEND_DEF "SC_GFX_BACKEND_D3D12")
+    set(SC_GFX_LIBS "")
+    set(SC_GFX_INCLUDES "")
+    message(STATUS "D3D12 backend selected (stub — link d3d12.lib on Windows)")
+elseif(SC_GFX_BACKEND STREQUAL "WGPU")
+    set(SC_GFX_BACKEND_DEF "SC_GFX_BACKEND_WGPU")
+    set(SC_GFX_LIBS "")
+    set(SC_GFX_INCLUDES "")
+    message(STATUS "WebGPU backend selected (stub — link wgpu-native or Dawn)")
 else()
     set(SC_GFX_BACKEND_DEF "SC_GFX_BACKEND_SOFTWARE")
     set(SC_GFX_LIBS "")

backends/sc_backend_d3d12.h

@@ -36,6 +36,7 @@ SCGfxShader   sc_d3d12_make_shader   (SCGfxContext *ctx, const SCGfxShaderDesc *
 void          sc_d3d12_destroy_shader(SCGfxContext *ctx, SCGfxShader shd);
 SCGfxPipeline sc_d3d12_make_pipeline (SCGfxContext *ctx, const SCGfxPipelineDesc *desc);
 void          sc_d3d12_destroy_pipeline(SCGfxContext *ctx, SCGfxPipeline pip);
+SCResult      sc_d3d12_resize         (SCGfxContext *ctx, u32 width, u32 height);
 
 #ifdef SC_BACKEND_D3D12_IMPLEMENTATION
 #include <stdio.h>
@@ -79,5 +80,9 @@ SCGfxPipeline sc_d3d12_make_pipeline(SCGfxContext *ctx, const SCGfxPipelineDesc
 void sc_d3d12_destroy_pipeline(SCGfxContext *ctx, SCGfxPipeline pip) {
     SC_UNUSED(ctx); SC_UNUSED(pip);
 }
+SCResult sc_d3d12_resize(SCGfxContext *ctx, u32 width, u32 height) {
+    SC_UNUSED(ctx); SC_UNUSED(width); SC_UNUSED(height);
+    return SC_ERR_NOT_SUPPORTED;
+}
 #endif /* SC_BACKEND_D3D12_IMPLEMENTATION */
 #endif /* SC_BACKEND_D3D12_H */