feat(shader): Add BlurPipeline class for frosted glass FBO setup

- Add BlurPipeline class with downsampled FBO support (scene, blur_a, blur_b)
- Implement 2-pass Gaussian blur shaders (horizontal + vertical)
- Add setup_fbos(), compile_blur_shaders(), prepare_blur(), cleanup() methods
- Add tests for BlurPipeline initialization, FBO setup, shader compilation, blur execution, and cleanup

Task: Phase 1, Task 1 of frosted_glass_20260313 track

conductor/tracks/frosted_glass_20260313/plan.md

@@ -1,7 +1,7 @@
 # Implementation Plan: Frosted Glass Background Effect (REPAIR - TRUE GPU)
 
 ## Phase 1: Robust Shader & FBO Foundation
-- [ ] Task: Implement: Create `ShaderManager` methods for downsampled FBO setup (scene, temp, blur).
+- [~] Task: Implement: Create `ShaderManager` methods for downsampled FBO setup (scene, temp, blur).
 - [ ] Task: Implement: Develop the "Deep Sea" background shader and integrate it as the FBO source.
 - [ ] Task: Implement: Develop the 2-pass Gaussian blur shaders with a wide tap distribution.
 - [ ] Task: Conductor - User Manual Verification 'Phase 1: Robust Foundation' (Protocol in workflow.md)