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)

scripts/game_systems/treatment_manager.gd

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+class_name TreatmentManager
+extends Node
+## Drives nausea during an active treatment.
+##
+## When `start_ramp()` is called (wired to IV `iv_completed`), nausea ramps from
+## its current value up to `ramp_target` over `ramp_duration` seconds, then holds.
+## The SymptomManager autoload propagates changes to the tilt + desaturation
+## effects, so this node only needs to push intensity values.
+
+signal treatment_started
+signal ramp_complete
+
+## Target nausea intensity at the end of the initial ramp.
+@export var ramp_target: float = 40.0
+## How long (seconds) the initial ramp takes.
+@export var ramp_duration: float = 30.0
+
+var _ramping: bool = false
+var _elapsed: float = 0.0
+var _start_value: float = 0.0
+var _sm: Node = null
+
+
+func _ready() -> void:
+	if is_inside_tree():
+		_sm = get_tree().root.get_node_or_null("SymptomManager")
+	if _sm == null:
+		_sm = SymptomManager
+	set_process(false)
+
+
+## Begin the nausea ramp. Safe to wire directly to IVInsertion.iv_completed.
+func start_ramp() -> void:
+	if _sm == null:
+		push_warning("TreatmentManager: SymptomManager not available.")
+		return
+	_start_value = _sm.get_intensity(_sm.NAUSEA)
+	_elapsed = 0.0
+	_ramping = true
+	set_process(true)
+	treatment_started.emit()
+
+
+func _process(delta: float) -> void:
+	if not _ramping:
+		return
+	_elapsed += delta
+	var t := clampf(_elapsed / ramp_duration, 0.0, 1.0)
+	var value := lerpf(_start_value, ramp_target, t)
+	_sm.trigger_symptom(_sm.NAUSEA, value)
+	if t >= 1.0:
+		_ramping = false
+		set_process(false)
+		ramp_complete.emit()