fix(tools): canyons impose a floor material through rock outcrops; tighten showcase contracts

Review findings: the mesa forced rock onto the canyon bed, two runners
declared the wrong TEST_COUNT, and the escarpment check could pass on the
slope alone. Canyon gains floor_material, the showcase is re-baked with sand
beds, the contract measures the step across the crest band, counts distinct
ledges, and probes the crevasse against the open plateau. Degenerate recipe
parameters now fail an assert.

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

tests/integration/run_site_terrain_integration.gd

@@ -240,7 +240,7 @@ func _test_showcase_site() -> void:
 	var mesa_top := _terrain_height(space, terrain, -80.0, -90.0, [])
 	_expect(mesa_top - canyon_floor >= 18.0, "collision rays find the canyon under the mesa (%.1f m vs %.1f m)" % [canyon_floor, mesa_top])
 
-	# Sensor above the canyon looking along its first leg (the acquisition probes along
+	# Sensor above the canyon looking along its second leg (the acquisition probes along
 	# local +Z): rock walls and sand floor.
 	var sensor := Transform3D(Basis.IDENTITY, Vector3(-130.0, -15.0, -135.0))
 	sensor.basis = Basis.looking_at(-Vector3(40.0, 0.0, -30.0).normalized(), Vector3.UP)