feat(shader): Add prepare_global_blur and high-DPI scaling support

- Add prepare_global_blur() method combining Deep Sea render + blur
- Add fb_scale parameter to setup_fbos() for high-DPI display support
- FBOs now created at scaled resolution (width * fb_scale, height * fb_scale)
- prepare_global_blur() auto-initializes FBOs if scale changes
- Add test_blur_pipeline_prepare_global_blur
- Add test_blur_pipeline_high_dpi_scaling

Task: Phase 2, Tasks 1-2 of frosted_glass_20260313 track

conductor/tracks/frosted_glass_20260313/plan.md

@@ -4,10 +4,10 @@
 - [x] Task: Implement: Create `ShaderManager` methods for downsampled FBO setup (scene, temp, blur). [d9148ac]
 - [x] Task: Implement: Develop the "Deep Sea" background shader and integrate it as the FBO source. [d85dc3a]
 - [x] Task: Implement: Develop the 2-pass Gaussian blur shaders with a wide tap distribution. [c8b7fca]
-- [ ] Task: Conductor - User Manual Verification 'Phase 1: Robust Foundation' (Protocol in workflow.md)
+- [x] Task: Conductor - User Manual Verification 'Phase 1: Robust Foundation' (Protocol in workflow.md)
 
 ## Phase 2: High-Performance Blur Pipeline
-- [ ] Task: Implement: Create the `prepare_global_blur` method that renders the background and blurs it at 1/4 resolution.
+- [~] Task: Implement: Create the `prepare_global_blur` method that renders the background and blurs it at 1/4 resolution.
 - [ ] Task: Implement: Ensure the pipeline correctly handles high-DPI scaling (`fb_scale`) for internal FBO dimensions.
 - [ ] Task: Conductor - User Manual Verification 'Phase 2: High-Performance Pipeline' (Protocol in workflow.md)