feat(tools): Strata modifier quantises steep walls into ledges

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 := 36
+const TEST_COUNT := 40
 
 var _failures: Array[String] = []
 
@@ -22,6 +22,7 @@ func _init() -> void:
 	_test_escarpment()
 	_test_outcrop()
 	_test_boulder_field()
+	_test_strata()
 
 	if _failures.is_empty():
 		print("Site baker unit tests: %d passed, 0 failed." % TEST_COUNT)
@@ -174,6 +175,43 @@ func _test_boulder_field() -> void:
 	_expect(stacked.elevation_delta(0.0, 0.0) <= 4.0 + 0.001, "overlapping boulders combine by maximum, not by sum")
 
 
+func _test_strata() -> void:
+	# A 45 degree plane sampled every 0.5 m: four cells per 2 m layer.
+	var recipe := _small_recipe()
+	recipe.columns = 41
+	recipe.rows = 41
+	recipe.cell_size_m = 0.5
+	recipe.origin_x_m = -10.0
+	recipe.origin_z_m = -10.0
+	recipe.primitives.append(SiteTerrainBaker.SlopedPlane.new(30.0, 1.0, 0.0, 0.0, 0.0))
+	var plain := SiteTerrainBaker.bake(recipe)
+	recipe.modifiers.append(SiteTerrainBaker.Strata.new(2.0, 0.9, 20.0, 35.0, Vector2.ZERO, 0.0))
+	var terraced := SiteTerrainBaker.bake(recipe)
+	var flat_cells := 0
+	var riser_cells := 0
+	for column in range(5, 35):
+		var rise := terraced.elevations_m[terraced.index_of(column + 1, 20)] - terraced.elevations_m[terraced.index_of(column, 20)]
+		if rise < 0.1:
+			flat_cells += 1
+		elif rise > 0.9:
+			riser_cells += 1
+	_expect(flat_cells >= 8 and riser_cells >= 4, "strata turn a steep slope into ledges and risers (%d flat, %d risers)" % [flat_cells, riser_cells])
+	_expect(is_equal_approx(terraced.elevations_m[terraced.index_of(20, 20)], plain.elevations_m[plain.index_of(20, 20)]), "strata keep the layer boundaries where they were")
+
+	var flat := _small_recipe()
+	flat.primitives.append(SiteTerrainBaker.SlopedPlane.new(30.3, 0.0, 0.0, 0.0, 0.0))
+	flat.modifiers.append(SiteTerrainBaker.Strata.new(2.0, 0.9, 20.0, 35.0, Vector2.ZERO, 0.0))
+	var flat_data := SiteTerrainBaker.bake(flat)
+	_expect(is_equal_approx(flat_data.elevations_m[flat_data.index_of(10, 10)], -30.3), "strata leave flat ground alone")
+
+	var identity := _small_recipe()
+	identity.primitives.append(SiteTerrainBaker.SlopedPlane.new(30.0, 1.0, 0.0, 0.0, 0.0))
+	var before := SiteTerrainBaker.bake(identity)
+	identity.modifiers.append(SiteTerrainBaker.Strata.new(2.0, 0.0, 20.0, 35.0, Vector2.ZERO, 0.0))
+	var after := SiteTerrainBaker.bake(identity)
+	_expect(before.elevations_m == after.elevations_m, "sharpness 0 is the identity")
+
+
 func _test_zone1_artefact_is_stable() -> void:
 	var versioned := load(ZONE1_TERRAIN_PATH) as SiteTerrainData
 	_expect(versioned != null, "versioned zone 1 terrain loads")