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.

scripts/game_systems/symptom_manager.gd

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+extends Node
+## Autoload singleton managing symptom states.
+##
+## Intensities are floats in the range [0, 100]. Nausea is the MVP symptom but
+## the API is generic so future symptoms (fatigue, taste distortion, etc.) can
+## reuse it. Register this script as an autoload named `SymptomManager`.
+
+## Emitted whenever any symptom intensity changes.
+## `symptom_id` is the changed symptom, `value` its new clamped intensity.
+signal symptom_intensity_changed(symptom_id: StringName, value: float)
+
+## Convenience signal specific to nausea (MVP visual effects subscribe to this).
+signal nausea_intensity_changed(value: float)
+
+const MIN_INTENSITY: float = 0.0
+const MAX_INTENSITY: float = 100.0
+
+## Well-known symptom ids.
+const NAUSEA: StringName = &"nausea"
+
+var _intensities: Dictionary = {}
+
+
+## Set a symptom's intensity to an absolute value (clamped to [0, 100]).
+func trigger_symptom(symptom_id: StringName, intensity: float) -> void:
+	_set_intensity(symptom_id, intensity)
+
+
+## Add (or subtract, with a negative delta) to a symptom's current intensity.
+func add_intensity(symptom_id: StringName, delta: float) -> void:
+	_set_intensity(symptom_id, get_intensity(symptom_id) + delta)
+
+
+## Return a symptom's current intensity, or 0.0 if never set.
+func get_intensity(symptom_id: StringName) -> float:
+	return _intensities.get(symptom_id, 0.0)
+
+
+## Reset all symptoms to 0 (e.g. between treatments / new game).
+func reset_all() -> void:
+	for symptom_id in _intensities.keys():
+		_set_intensity(symptom_id, 0.0)
+
+
+func _set_intensity(symptom_id: StringName, value: float) -> void:
+	var clamped := clampf(value, MIN_INTENSITY, MAX_INTENSITY)
+	if is_equal_approx(_intensities.get(symptom_id, 0.0), clamped):
+		return
+	_intensities[symptom_id] = clamped
+	symptom_intensity_changed.emit(symptom_id, clamped)
+	if symptom_id == NAUSEA:
+		nausea_intensity_changed.emit(clamped)