Unified PSO validation semantics across D3D12 and OpenGL backends:
- IsValid() returns whether PSO is ready to use
- EnsureValid() ensures PSO is valid (compiles if needed)
Behavior by backend:
- D3D12: IsValid=false after creation, true after EnsureValid() with shaders
- OpenGL: IsValid always=true (immediate model)
Also added test_pipeline_state.cpp with 10 tests for RHIPipelineState.
