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.

Entities/GroundTile/scripts/bush_multimesh.gd

@@ -0,0 +1,56 @@
+extends MultiMeshInstance3D
+var mm: MultiMesh
+var parent_node: BushController
+static var bush_mesh: Mesh = null
+var material: ShaderMaterial
+
+func _ready() -> void:
+	parent_node = get_parent() as BushController
+	if parent_node == null:
+		Log.pr("Error: Parent node is not a BushController!")
+	
+	# Load mesh once and reuse
+	if bush_mesh == null:
+		bush_mesh = load("res://Entities/Bush/assets/bush_large.res")
+
+func setup_multimesh() -> void:
+	if parent_node == null:
+		return
+	
+	if bush_mesh == null:
+		Log.pr("Error: Could not load bush mesh")
+		return
+	
+	# Reuse existing MultiMesh if possible, or create new one
+	if mm == null:
+		mm = MultiMesh.new()
+		mm.transform_format = MultiMesh.TRANSFORM_3D
+		mm.mesh = bush_mesh
+
+		material = ColorData.get_random_bush_material(Season.current)
+		mm.mesh.surface_set_material(0, material)
+	
+	# Configure instance count
+	mm.instance_count = parent_node.bush_instance_range
+	
+	# Get shared RNG from GroundTile
+	var rng = parent_node.parent_node.get_rng()
+	
+	# Generate positions using shared RNG
+	for i in range(mm.instance_count):
+		var random_pos = Vector3(
+			rng.randf_range(-1.0, 1.0),
+			0.0,
+			rng.randf_range(-1.0, 1.0)
+		)
+		
+		var random_rotation = rng.randf_range(0.0, TAU)
+		var basis = Basis(Vector3.UP, random_rotation)
+		
+		var random_scale = rng.randf_range(0.3, 0.9)
+		basis = basis.scaled(Vector3(random_scale, random_scale, random_scale))
+		
+		var tx = Transform3D(basis, random_pos)
+		mm.set_instance_transform(i, tx)
+
+	multimesh = mm