feat(tools): modifier phase and per-primitive material hook in the site baker

The bake order is now primitive sum, modifiers (with the local gradient),
floor clamp, slope material, primitive materials, material patches. A
regression test checks the Banc du Vigilant still bakes byte for byte.

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

tools/site_terrain_baker.gd

@@ -5,10 +5,19 @@ const SiteTerrainData = preload("res://game/domain/sites/site_terrain_data.gd")
 const TerrainHeightField = preload("res://game/domain/sites/terrain_height_field.gd")
 
 
+## A primitive returning this from material_at leaves the slope rule in charge.
+const NO_MATERIAL := -1
+
+
 class Primitive extends RefCounted:
 	func elevation_delta(_x_m: float, _z_m: float) -> float:
 		return 0.0
 
+	## Material imposed by this primitive at (x, z), or NO_MATERIAL. Applied after the
+	## slope rule, in recipe order, before the material patches.
+	func material_at(_x_m: float, _z_m: float) -> int:
+		return NO_MATERIAL
+
 	static func distance_to_segment(point: Vector2, start: Vector2, end: Vector2) -> float:
 		var segment := end - start
 		var length_squared := segment.length_squared()
@@ -171,6 +180,13 @@ class Roughness extends Primitive:
 		return _noise.get_noise_2d(x_m, z_m) * amplitude_m
 
 
+## Reshapes the summed elevation once every primitive has been added. gradient is the
+## local slope Vector2(dE/dx, dE/dz) of the field being modified, by central differences.
+class Modifier extends RefCounted:
+	func modify(_x_m: float, _z_m: float, elevation_m: float, _gradient: Vector2) -> float:
+		return elevation_m
+
+
 class MaterialPatch extends RefCounted:
 	var centre: Vector2
 	var radius_m: float
@@ -193,9 +209,12 @@ class Recipe extends RefCounted:
 	var rock_reflectivity: float = 0.72
 	var sand_reflectivity: float = 0.45
 	var primitives: Array[Primitive] = []
+	var modifiers: Array[Modifier] = []
 	var material_patches: Array[MaterialPatch] = []
 
 
+## Bake order: primitive sum, modifiers in order, floor clamp, slope material,
+## primitive materials in order, material patches.
 static func bake(recipe: Recipe) -> SiteTerrainData:
 	var data := SiteTerrainData.new()
 	data.columns = recipe.columns
@@ -218,8 +237,12 @@ static func bake(recipe: Recipe) -> SiteTerrainData:
 			var elevation := 0.0
 			for primitive: Primitive in recipe.primitives:
 				elevation += primitive.elevation_delta(x_m, z_m)
-			elevations[data.index_of(column, row)] = maxf(elevation, recipe.floor_elevation_m)
-			materials[data.index_of(column, row)] = SonarMaterial.Value.SAND
+			elevations[data.index_of(column, row)] = elevation
+	for modifier: Modifier in recipe.modifiers:
+		elevations = _apply_modifier(recipe, modifier, elevations)
+	for index in cell_count:
+		elevations[index] = maxf(elevations[index], recipe.floor_elevation_m)
+		materials[index] = SonarMaterial.Value.SAND
 	data.elevations_m = elevations
 	data.materials = materials
 	var field := TerrainHeightField.from_data(data)
@@ -230,6 +253,14 @@ static func bake(recipe: Recipe) -> SiteTerrainData:
 		for column in recipe.columns:
 			if field.normal_at_cell(column, row).y < rock_cosine:
 				materials[data.index_of(column, row)] = SonarMaterial.Value.ROCK
+	for primitive: Primitive in recipe.primitives:
+		for row in recipe.rows:
+			var z_m := recipe.origin_z_m + float(row) * recipe.cell_size_m
+			for column in recipe.columns:
+				var x_m := recipe.origin_x_m + float(column) * recipe.cell_size_m
+				var material := primitive.material_at(x_m, z_m)
+				if material != NO_MATERIAL:
+					materials[data.index_of(column, row)] = material
 	for patch: MaterialPatch in recipe.material_patches:
 		for row in recipe.rows:
 			var z_m := recipe.origin_z_m + float(row) * recipe.cell_size_m
@@ -241,6 +272,31 @@ static func bake(recipe: Recipe) -> SiteTerrainData:
 	return data
 
 
+## Returns the modified copy of elevations; the gradient of each cell is read from the
+## unmodified input so the modifier sees one consistent field.
+static func _apply_modifier(recipe: Recipe, modifier: Modifier, source: PackedFloat32Array) -> PackedFloat32Array:
+	var columns := recipe.columns
+	var rows := recipe.rows
+	var cell_size := recipe.cell_size_m
+	var result := PackedFloat32Array()
+	result.resize(source.size())
+	for row in rows:
+		var z_m := recipe.origin_z_m + float(row) * cell_size
+		var north := maxi(row - 1, 0)
+		var south := mini(row + 1, rows - 1)
+		for column in columns:
+			var x_m := recipe.origin_x_m + float(column) * cell_size
+			var west := maxi(column - 1, 0)
+			var east := mini(column + 1, columns - 1)
+			var gradient := Vector2(
+				(source[row * columns + east] - source[row * columns + west]) / (float(east - west) * cell_size),
+				(source[south * columns + column] - source[north * columns + column]) / (float(south - north) * cell_size)
+			)
+			var index := row * columns + column
+			result[index] = modifier.modify(x_m, z_m, source[index], gradient)
+	return result
+
+
 static func write_preview_png(data: SiteTerrainData, path: String) -> Error:
 	var field := TerrainHeightField.from_data(data)
 	if field == null: