Detail transition to quaternions

Author: llGaetanll

TODO.md

@@ -70,10 +70,26 @@ Get input handling working and validate the camera math before touching the shad
 
 Once CPU-side camera is working:
 
-- [ ] Add camera fields to `ShaderConstants` (cam_pos, cam_dir, cam_fov, etc.)
-- [ ] Modify shader to construct `Camera` from `ShaderConstants` instead of hardcoded values
+- [x] Add camera fields to `ShaderConstants` (cam_pos, cam_dir, cam_fov, etc.)
+- [x] Modify shader to construct `Camera` from `ShaderConstants` instead of hardcoded values
 - [ ] Remove or bypass per-scene camera construction
 
+### Phase 3: Fix gimbal lock crash
+
+Camera crashes when looking straight up or down due to `vup` being parallel to view direction.
+
+**Stage 1: Add `vup` to data path**
+- [ ] Add `vup: [f32; 3]` to `ShaderConstants`
+- [ ] Pass `vup` through to `CameraParams` and `Camera::new()`
+- [ ] Host computes `vup` dynamically (switch reference vector when pitch > 45°)
+- [ ] Validate fix: full vertical range without crash
+
+**Stage 2: Switch host to quaternions**
+- [ ] Replace `cam_yaw`/`cam_pitch` with `glam::Quat` orientation
+- [ ] Rewrite `update_camera()` to apply incremental quaternion rotations
+- [ ] Extract `cam_dir` and `vup` from quaternion for shader
+- [ ] Tune input feel (yaw in world space vs local space)
+
 ### Future enhancements (not for first pass)
 
 - Sample accumulation when stationary