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>

tests/unit/run_site_baker_tests.gd

@@ -3,8 +3,12 @@ extends SceneTree
 const SonarMaterial = preload("res://game/domain/sonar/sonar_material.gd")
 const SiteTerrainBaker = preload("res://tools/site_terrain_baker.gd")
 const VehicleTestRecipe = preload("res://tools/sites/vehicle_test_terrain_recipe.gd")
+const Zone1Recipe = preload("res://tools/sites/zone1_vigilant_recipe.gd")
+const SiteTerrainData = preload("res://game/domain/sites/site_terrain_data.gd")
 
-const TEST_COUNT := 10
+const ZONE1_TERRAIN_PATH := "res://content/sites/zone1_vigilant_terrain.res"
+
+const TEST_COUNT := 16
 
 var _failures: Array[String] = []
 
@@ -12,6 +16,8 @@ var _failures: Array[String] = []
 func _init() -> void:
 	_test_plane_and_blob()
 	_test_materials_and_determinism()
+	_test_modifiers_and_primitive_materials()
+	_test_zone1_artefact_is_stable()
 
 	if _failures.is_empty():
 		print("Site baker unit tests: %d passed, 0 failed." % TEST_COUNT)
@@ -66,6 +72,59 @@ func _test_materials_and_determinism() -> void:
 	_expect(calibration != null and calibration.validation_error().is_empty(), "calibration recipe bakes valid data")
 
 
+## Test doubles for the extension points: a modifier that lifts the field by one metre
+## and a primitive that forces rock on a flat plane.
+class Lift extends SiteTerrainBaker.Modifier:
+	func modify(_x_m: float, _z_m: float, elevation_m: float, _gradient: Vector2) -> float:
+		return elevation_m + 1.0
+
+
+class GradientProbe extends SiteTerrainBaker.Modifier:
+	var gradient_at_centre := Vector2.ZERO
+
+	func modify(x_m: float, z_m: float, elevation_m: float, gradient: Vector2) -> float:
+		if is_zero_approx(x_m) and is_zero_approx(z_m):
+			gradient_at_centre = gradient
+		return elevation_m
+
+
+class RockDisc extends SiteTerrainBaker.Primitive:
+	func material_at(x_m: float, z_m: float) -> int:
+		if Vector2(x_m, z_m).length() <= 5.0:
+			return SonarMaterial.Value.ROCK
+		return SiteTerrainBaker.NO_MATERIAL
+
+
+func _test_modifiers_and_primitive_materials() -> void:
+	var recipe := _small_recipe()
+	recipe.primitives.append(SiteTerrainBaker.SlopedPlane.new(30.0, 0.2, 0.0, 0.0, 0.0))
+	var probe := GradientProbe.new()
+	recipe.modifiers.append(probe)
+	recipe.modifiers.append(Lift.new())
+	var data := SiteTerrainBaker.bake(recipe)
+	_expect(is_equal_approx(data.elevations_m[data.index_of(10, 10)], -29.0), "modifier runs after the primitive sum")
+	_expect(probe.gradient_at_centre.is_equal_approx(Vector2(0.2, 0.0)), "modifier receives the local gradient (%s)" % probe.gradient_at_centre)
+
+	var flat := _small_recipe()
+	flat.primitives.append(SiteTerrainBaker.SlopedPlane.new(30.0, 0.0, 0.0, 0.0, 0.0))
+	flat.primitives.append(RockDisc.new())
+	var flat_data := SiteTerrainBaker.bake(flat)
+	_expect(flat_data.materials[flat_data.index_of(10, 10)] == SonarMaterial.Value.ROCK, "primitive material wins over the slope rule")
+	_expect(flat_data.materials[flat_data.index_of(0, 0)] == SonarMaterial.Value.SAND, "primitive material is local")
+
+
+func _test_zone1_artefact_is_stable() -> void:
+	var versioned := load(ZONE1_TERRAIN_PATH) as SiteTerrainData
+	_expect(versioned != null, "versioned zone 1 terrain loads")
+	if versioned == null:
+		return
+	var baked := SiteTerrainBaker.bake(Zone1Recipe.recipe())
+	_expect(
+		baked.elevations_m == versioned.elevations_m and baked.materials == versioned.materials,
+		"zone 1 recipe still bakes the versioned artefact byte for byte"
+	)
+
+
 func _expect(condition: bool, label: String) -> void:
 	if not condition:
 		_failures.append(label)