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/character/iv_insertion.gd

@@ -0,0 +1,62 @@
+class_name IVInsertion
+extends Node3D
+## IV insertion sequence. Presents an IV arm in front of the seated player,
+## plays a short needle-approach + tape-press animation, then emits
+## `iv_completed` which starts the treatment timer.
+
+signal iv_completed
+
+@export var auto_hide_on_complete: bool = true
+@export var needle_approach_time: float = 1.2
+@export var tape_press_time: float = 0.6
+@export var hold_time: float = 0.5
+
+@onready var needle: Node3D = $Arm/Needle
+@onready var tape: Node3D = $Arm/Tape
+
+var _needle_start: Vector3
+var _needle_end: Vector3
+var _tape_hidden_scale: Vector3 = Vector3(1, 0.01, 1)
+var _tape_shown_scale: Vector3 = Vector3.ONE
+var _running: bool = false
+
+
+func _ready() -> void:
+	visible = false
+	if needle:
+		_needle_start = needle.position
+		# Needle ends pressed into the arm (move toward the arm along +Z/-Y a touch).
+		_needle_end = needle.position + Vector3(0, -0.04, -0.08)
+	if tape:
+		tape.scale = _tape_hidden_scale
+
+
+## Begin the IV insertion sequence. Safe to wire directly to `player_seated`.
+func begin_sequence() -> void:
+	if _running:
+		return
+	_running = true
+	visible = true
+	if needle:
+		needle.position = _needle_start
+	if tape:
+		tape.scale = _tape_hidden_scale
+
+	var tween := create_tween()
+	tween.set_trans(Tween.TRANS_SINE).set_ease(Tween.EASE_IN_OUT)
+	# Needle approaches and presses in.
+	if needle:
+		tween.tween_property(needle, "position", _needle_end, needle_approach_time)
+	# Tape presses on.
+	if tape:
+		tween.tween_property(tape, "scale", _tape_shown_scale, tape_press_time)
+	# Brief hold, then finish.
+	tween.tween_interval(hold_time)
+	tween.tween_callback(_on_sequence_finished)
+
+
+func _on_sequence_finished() -> void:
+	_running = false
+	if auto_hide_on_complete:
+		visible = false
+	iv_completed.emit()