Set up framework for more advance pickup behaviours.

resources/pickups/pickup_drift_behavior.gd

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+class_name PickupDriftBehavior extends Resource
+
+## Base resource that defines the global travel pattern of a pickup across
+## the screen. Assign to [member drift_behavior] on a PickupBehaviorController.
+
+enum DriftType {
+	STATIONARY, ## Does not move (aside from bob/shake).
+	SINE_WAVE, ## Travels in a sine curve.
+	LEAF_FALL, ## Drifts downward with gentle horizontal sway.
+	DIAGONAL, ## Moves diagonally in one direction.
+}
+
+@export_group("Drift")
+## The drift pattern to apply.
+@export var drift_type: DriftType = DriftType.STATIONARY
+
+## Horizontal speed in pixels per second (applies to SINE_WAVE, LEAF_FALL).
+@export var drift_speed: float = 30.0
+## Vertical fall speed in pixels per second (applies to LEAF_FALL).
+@export var fall_speed: float = 15.0
+
+## Horizontal wave amplitude in pixels (applies to SINE_WAVE, LEAF_FALL).
+@export var wave_amplitude: float = 40.0
+## Wave frequency in cycles per second (applies to SINE_WAVE, LEAF_FALL).
+@export var wave_frequency: float = 1.0
+
+## Diagonal direction: +X forward, -X backward (applies to DIAGONAL).
+@export var diagonal_forward: bool = true
+
+## Main drift axis for SINE_WAVE and LEAF_FALL.
+## true = horizontal travel (x), false = vertical travel (y).
+@export var drift_along_x: bool = true
+
+## Reverse the primary direction (left/right or up/down).
+@export var reverse_direction: bool = false
+
+## Randomize the perpendicular axis origin (y if horizontal, x if vertical).
+@export var randomize_origin: bool = false
+
+## Phase offset in radians — randomize per-instance for variety.
+@export var phase_offset: float = 0.0
+
+var _initial_x: float = 0.0
+
+
+func reset(initial_position: Vector2) -> void:
+	_initial_x = initial_position.x