Adds bushes and flowers to ground tiles

Implements bush and flower spawning on ground tiles based on vegetation density.
Adds new assets for bushes and flowers, and introduces multi-mesh rendering for optimized performance.
Introduces seasonal color variations for vegetation using a shader for bushes and materials for flowers and grass.
Refactors material application into a MaterialManager to handle material assignments over multiple frames.
Moves ground tile scripts into a subfolder.
Adds floating particles to test scene.

stages/Test3D/particles.gd

@@ -0,0 +1,79 @@
+extends GPUParticles3D
+
+@onready var player: Node3D = get_node("%Player")
+var last_player_position: Vector3
+var update_distance: float = 1
+
+func _ready():
+	setup_floating_particles()
+
+func _process(delta):
+	if player:
+		var current_player_pos = player.global_position
+
+		var target_pos = Vector3(
+			current_player_pos.x,
+			1.0,
+			current_player_pos.z
+		)
+
+		global_position = global_position.lerp(target_pos, delta * 3.0)
+
+func setup_floating_particles():
+	# Basic particle setup
+	emitting = true
+	amount = 100
+	lifetime = 8.0
+	visibility_aabb = AABB(Vector3(-20, 0, -20), Vector3(40, 10, 40))
+	
+	# Create material
+	var material = ParticleProcessMaterial.new()
+	
+	# Emission
+	material.emission_shape = ParticleProcessMaterial.EMISSION_SHAPE_BOX
+	material.emission_box_extents = Vector3(5, 2, 5)
+	
+	# Movement
+	material.direction = Vector3(0.1, 0.8, 0.1)
+	material.initial_velocity_min = 0.2
+	material.initial_velocity_max = 0.8
+	material.gravity = Vector3(0, -0.3, 0)
+	
+	# Floating motion
+	material.orbit_velocity_min = 0.1
+	material.orbit_velocity_max = 0.3
+	material.radial_velocity_min = -0.2
+	material.radial_velocity_max = 0.2
+	
+	# Size and fade
+	material.scale_min = 0.01
+	material.scale_max = 0.03
+	material.scale_over_velocity_min = 0.0
+	material.scale_over_velocity_max = 2.0
+	
+	# Color (golden dust particles)
+	var gradient = Gradient.new()
+	gradient.add_point(0.0, Color(1.0, 0.9, 0.6, 0.0)) # Fade in
+	gradient.add_point(0.2, Color(1.0, 0.9, 0.6, 0.5)) # Full opacity
+	gradient.add_point(0.8, Color(1.0, 0.8, 0.4, 0.3)) # Slight color shift
+	gradient.add_point(1.0, Color(1.0, 0.7, 0.3, 0.0)) # Fade out
+	
+	var gradient_texture = GradientTexture1D.new()
+	gradient_texture.gradient = gradient
+	material.color_ramp = gradient_texture
+	
+	# Assign the material to the particle system
+	process_material = material
+	
+	# Create and assign visual mesh (small billboard)
+	var quad_mesh = QuadMesh.new()
+	quad_mesh.size = Vector2(0.01, 0.01)
+	draw_pass_1 = quad_mesh
+	
+	# Create a basic material for the particles to be visible
+	var particle_material = StandardMaterial3D.new()
+	particle_material.albedo_color = ColorData.grass_materials[Season.current]['top']
+	particle_material.flags_transparent = true
+	particle_material.flags_unshaded = true
+	particle_material.billboard_mode = BaseMaterial3D.BILLBOARD_ENABLED
+	material_override = particle_material