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/unit/run_site_baker_tests.gd

@@ -8,7 +8,7 @@ const SiteTerrainData = preload("res://game/domain/sites/site_terrain_data.gd")
 
 const ZONE1_TERRAIN_PATH := "res://content/sites/zone1_vigilant_terrain.res"
 
-const TEST_COUNT := 40
+const TEST_COUNT := 44
 
 var _failures: Array[String] = []
 
@@ -121,16 +121,18 @@ func _test_modifiers_and_primitive_materials() -> void:
 func _test_canyon() -> void:
 	var canyon := SiteTerrainBaker.Canyon.new(
 		PackedVector2Array([Vector2(-20.0, 0.0), Vector2(0.0, 0.0), Vector2(0.0, 20.0)]),
-		4.0, 10.0, 1.0, 0, 0.0
+		4.0, 10.0, 1.0, 0, 0.0, SonarMaterial.Value.SAND
 	)
 	_expect(is_equal_approx(canyon.elevation_delta(-10.0, 0.0), -10.0), "canyon floor sits at depth on the first leg")
+	_expect(canyon.material_at(-10.0, 0.0) == SonarMaterial.Value.SAND, "canyon imposes its floor material on the flat floor")
+	_expect(canyon.material_at(-10.0, 4.0) == SiteTerrainBaker.NO_MATERIAL, "canyon leaves its walls to the slope rule")
 	_expect(is_equal_approx(canyon.elevation_delta(1.5, 10.0), -10.0), "canyon floor is flat across its width on the second leg")
 	_expect(is_equal_approx(canyon.elevation_delta(-10.0, 6.0), 0.0), "canyon stops past half width plus softness")
 	_expect(is_equal_approx(canyon.elevation_delta(15.0, -15.0), 0.0), "canyon is zero far from the path")
 	_expect(is_equal_approx(canyon.elevation_delta(0.0, 20.0), 0.0), "canyon closes at its last vertex")
 	_expect(canyon.elevation_delta(-10.0, 4.0) < -4.0 and canyon.elevation_delta(-10.0, 4.0) > -6.0, "canyon wall is half way down at half width")
 	var rough := SiteTerrainBaker.Canyon.new(
-		PackedVector2Array([Vector2(-200.0, 0.0), Vector2(200.0, 0.0)]), 8.0, 10.0, 2.0, 7, 3.0
+		PackedVector2Array([Vector2(-200.0, 0.0), Vector2(200.0, 0.0)]), 8.0, 10.0, 2.0, 7, 3.0, SiteTerrainBaker.NO_MATERIAL
 	)
 	var wall_values: Dictionary = {}
 	for x in range(-100, 101, 10):