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/treatment_manager.gd

@@ -0,0 +1,54 @@
+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()