feat(content): bake the Jardin des Formes terrain showcase site

Five reference landforms on one 512 m field at 1 m per cell: a stratified
canyon through a mesa, a crevasse between two buttes, a boulder field on
rippled sand, two rock spurs framing a sand channel, and a sinuous
escarpment with knolls on the slope below.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tools/bake_site_terrain.gd

@@ -5,6 +5,7 @@ const SiteTerrainBaker = preload("res://tools/site_terrain_baker.gd")
 const RECIPES := {
 	"vehicle_test": "res://tools/sites/vehicle_test_terrain_recipe.gd",
 	"zone1_vigilant": "res://tools/sites/zone1_vigilant_recipe.gd",
+	"terrain_showcase": "res://tools/sites/terrain_showcase_recipe.gd",
 }
 const OUTPUT_DIRECTORY := "res://content/sites"
 const PREVIEW_DIRECTORY := "res://docs/briefs/captures"