feat(sites): typed terrain data and height field sampler

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

game/domain/sites/site_terrain_data.gd

@@ -0,0 +1,44 @@
+class_name SiteTerrainData
+extends Resource
+
+const SonarMaterial = preload("res://game/domain/sonar/sonar_material.gd")
+
+## Regular single-surface heightfield. Column grows with world +X (east),
+## row grows with world +Z (south). Elevations are world Y in metres.
+@export var columns: int = 0
+@export var rows: int = 0
+@export var cell_size_m: float = 1.0
+@export var origin_x_m: float = 0.0
+@export var origin_z_m: float = 0.0
+@export var elevations_m := PackedFloat32Array()
+@export var materials := PackedByteArray()
+@export var rock_reflectivity: float = 0.72
+@export var sand_reflectivity: float = 0.45
+
+
+func index_of(column: int, row: int) -> int:
+	return row * columns + column
+
+
+func validation_error() -> String:
+	if columns < 2 or rows < 2:
+		return "terrain needs at least 2 columns and 2 rows"
+	if not is_finite(cell_size_m) or cell_size_m <= 0.0:
+		return "terrain cell size must be positive"
+	if not is_finite(origin_x_m) or not is_finite(origin_z_m):
+		return "terrain origin must be finite"
+	var cell_count := columns * rows
+	if elevations_m.size() != cell_count:
+		return "terrain elevation count does not match the grid"
+	if materials.size() != cell_count:
+		return "terrain material count does not match the grid"
+	for elevation: float in elevations_m:
+		if not is_finite(elevation):
+			return "terrain elevations must be finite"
+	for material: int in materials:
+		if not SonarMaterial.is_terrain(material):
+			return "terrain materials must be rock or sand"
+	for reflectivity: float in [rock_reflectivity, sand_reflectivity]:
+		if not is_finite(reflectivity) or reflectivity <= 0.0 or reflectivity > 1.0:
+			return "terrain reflectivity must be within ]0, 1]"
+	return ""