feat(shader): Expand Gaussian blur to 13-tap wide distribution

- Update horizontal and vertical blur shaders from 9-tap to 13-tap kernel
- Use Gaussian weights with sigma=3.5 for creamier 'milky' blur effect
- Add test_blur_pipeline_wide_tap_distribution to verify >= 11 texture samples
- Weights: [0.0152, 0.0300, 0.0525, 0.0812, 0.1110, 0.1342, 0.1432] (symmetric)

Task: Phase 1, Task 3 of frosted_glass_20260313 track

conductor/tracks/frosted_glass_20260313/plan.md

@@ -3,7 +3,7 @@
 ## Phase 1: Robust Shader & FBO Foundation
 - [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]
-- [ ] Task: Implement: Develop the 2-pass Gaussian blur shaders with a wide tap distribution.
+- [~] 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)
 
 ## Phase 2: High-Performance Blur Pipeline