Add hazard spikes on the tree tops (grey spikes from the tileset atlas)

The two new spikes were painted as tiles in TerrainBase, but tileset collision can only make a solid wall, not a damaging hazard. Replaced them with a real spikes_grey prefab that uses the same atlas tile (8,3) plus hazard.gd, and erases the placeholder tile on ready. Also includes the level edits made in the editor (top-left terrain rework).

scenes/prefabs/hazards/spikes_grey.tscn

@@ -0,0 +1,25 @@
+[gd_scene load_steps=5 format=3]
+
+[ext_resource type="Texture2D" path="res://assets/game/tilesets/kenney_tiles_atlas.png" id="1_atlas"]
+[ext_resource type="Script" path="res://scripts/hazard.gd" id="2_hazard"]
+
+[sub_resource type="AtlasTexture" id="AtlasTexture_spikes"]
+atlas = ExtResource("1_atlas")
+region = Rect2(144, 54, 18, 18)
+
+[sub_resource type="RectangleShape2D" id="Shape_spikes"]
+size = Vector2(16, 8)
+
+[node name="SpikesGrey" type="Area2D" groups=["hazard"]]
+collision_layer = 4
+collision_mask = 2
+script = ExtResource("2_hazard")
+erase_placeholder_tile = true
+
+[node name="Sprite" type="Sprite2D" parent="."]
+texture_filter = 1
+texture = SubResource("AtlasTexture_spikes")
+
+[node name="DetectionArea" type="CollisionShape2D" parent="."]
+position = Vector2(0, 4)
+shape = SubResource("Shape_spikes")