feat: Implement nausea symptom system and related features

- Added SymptomManager autoload for managing symptom intensities.
- Introduced nausea tilt effect for camera roll based on nausea intensity.
- Created color desaturation shader and overlay for visual feedback.
- Developed TreatmentManager to handle nausea ramping during IV treatment.
- Added hospital room scene with first-person controller and interactions.
- Implemented sit-down interaction for the treatment chair.
- Created IV insertion sequence with animations and completion signal.
- Added player controller with movement and interaction capabilities.
- Integrated nausea effects into the player experience with smooth transitions.

assets/shaders/desaturation.gdshader

@@ -0,0 +1,17 @@
+shader_type canvas_item;
+// Full-screen nausea desaturation.
+// Samples the screen, converts to luminance, and blends back toward grayscale
+// based on the `desaturation` uniform (0 = full color, 1 = full grayscale).
+
+uniform sampler2D screen_tex : hint_screen_texture, filter_linear_mipmap;
+uniform float desaturation : hint_range(0.0, 1.0) = 0.0;
+
+// Rec. 709 luma weights.
+const vec3 LUMA = vec3(0.2126, 0.7152, 0.0722);
+
+void fragment() {
+	vec3 col = texture(screen_tex, SCREEN_UV).rgb;
+	float gray = dot(col, LUMA);
+	vec3 result = mix(col, vec3(gray), clamp(desaturation, 0.0, 1.0));
+	COLOR = vec4(result, 1.0);
+}