feat(shader): Add Deep Sea background shader for BlurPipeline

- Add compile_deepsea_shader() with animated underwater-like GLSL shader
- Add render_deepsea_to_fbo() to render background to scene FBO
- Deep Sea shader features: FBM noise, animated blobs, caustic lines, vignette
- Add deepsea_program to cleanup() for proper resource management
- Add 2 new tests for Deep Sea shader compilation and FBO rendering

Task: Phase 1, Task 2 of frosted_glass_20260313 track

conductor/tracks/frosted_glass_20260313/plan.md

@@ -2,7 +2,7 @@
 
 ## Phase 1: Robust Shader & FBO Foundation
 - [x] Task: Implement: Create `ShaderManager` methods for downsampled FBO setup (scene, temp, blur). [d9148ac]
-- [ ] Task: Implement: Develop the "Deep Sea" background shader and integrate it as the FBO source.
+- [~] 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)