perf(sites): build terrain tiles from bulk field blocks

Building the 64 tiles of the Banc du Vigilant cost about 1.9 s through
per-vertex cell_world_position/normal_at_cell calls, leaving the 3 s
load budget with too little headroom (2.1-2.9 s measured, one 4.8 s
outlier). TerrainHeightField now returns positions and normals of a
cell block in one call, with normal_at_cell sharing the same formula;
the tile build drops to about 0.5 s.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

game/domain/sites/terrain_height_field.gd

@@ -81,15 +81,50 @@ func cell_world_position(column: int, row: int) -> Vector3:
 
 
 func normal_at_cell(column: int, row: int) -> Vector3:
-	var left := elevation_at_cell(column - 1, row)
-	var right := elevation_at_cell(column + 1, row)
-	var north := elevation_at_cell(column, row - 1)
-	var south := elevation_at_cell(column, row + 1)
-	var span_x := float(_clamp_column(column + 1) - _clamp_column(column - 1)) * _data.cell_size_m
-	var span_z := float(_clamp_row(row + 1) - _clamp_row(row - 1)) * _data.cell_size_m
-	var slope_x := (right - left) / span_x
-	var slope_z := (south - north) / span_z
-	return Vector3(-slope_x, 1.0, -slope_z).normalized()
+	return _normal_at(_data.elevations_m, _clamp_column(column), _clamp_row(row))
+
+
+## World positions of the cells [start_column..end_column] x [start_row..end_row]
+## (bounds included, clamped to the grid), row-major. Same values as cell_world_position.
+func cell_block_positions(start_column: int, end_column: int, start_row: int, end_row: int) -> PackedVector3Array:
+	var first_column := _clamp_column(start_column)
+	var last_column := _clamp_column(end_column)
+	var first_row := _clamp_row(start_row)
+	var last_row := _clamp_row(end_row)
+	var elevations := _data.elevations_m
+	var columns := _data.columns
+	var cell_size := _data.cell_size_m
+	var positions := PackedVector3Array()
+	positions.resize((last_column - first_column + 1) * (last_row - first_row + 1))
+	var index := 0
+	for row in range(first_row, last_row + 1):
+		var z := _data.origin_z_m + float(row) * cell_size
+		var row_start := row * columns
+		for column in range(first_column, last_column + 1):
+			positions[index] = Vector3(
+				_data.origin_x_m + float(column) * cell_size,
+				elevations[row_start + column],
+				z
+			)
+			index += 1
+	return positions
+
+
+## Normals of the same cell block, row-major. Same values as normal_at_cell.
+func cell_block_normals(start_column: int, end_column: int, start_row: int, end_row: int) -> PackedVector3Array:
+	var first_column := _clamp_column(start_column)
+	var last_column := _clamp_column(end_column)
+	var first_row := _clamp_row(start_row)
+	var last_row := _clamp_row(end_row)
+	var elevations := _data.elevations_m
+	var normals := PackedVector3Array()
+	normals.resize((last_column - first_column + 1) * (last_row - first_row + 1))
+	var index := 0
+	for row in range(first_row, last_row + 1):
+		for column in range(first_column, last_column + 1):
+			normals[index] = _normal_at(elevations, column, row)
+			index += 1
+	return normals
 
 
 func elevation_at(x_m: float, z_m: float) -> float:
@@ -130,6 +165,24 @@ func reflectivity_for(material: int) -> float:
 			return _data.rock_reflectivity
 
 
+## Central differences, one-sided on the grid edges. column and row must be in range.
+func _normal_at(elevations: PackedFloat32Array, column: int, row: int) -> Vector3:
+	var columns := _data.columns
+	var left := maxi(column - 1, 0)
+	var right := mini(column + 1, columns - 1)
+	var north := maxi(row - 1, 0)
+	var south := mini(row + 1, _data.rows - 1)
+	var slope_x := (
+		(elevations[row * columns + right] - elevations[row * columns + left])
+		/ (float(right - left) * _data.cell_size_m)
+	)
+	var slope_z := (
+		(elevations[south * columns + column] - elevations[north * columns + column])
+		/ (float(south - north) * _data.cell_size_m)
+	)
+	return Vector3(-slope_x, 1.0, -slope_z).normalized()
+
+
 func _clamp_column(column: int) -> int:
 	return clampi(column, 0, _data.columns - 1)
 

game/world/sites/bathymetric_terrain_actor.gd

@@ -78,14 +78,12 @@ func _build_tile(start_column: int, end_column: int, start_row: int, end_row: in
 		0.0,
 		(corner_min.z + corner_max.z) * 0.5
 	)
-	var vertices := PackedVector3Array()
-	var normals := PackedVector3Array()
+	var vertices := _field.cell_block_positions(start_column, end_column, start_row, end_row)
+	for index in vertices.size():
+		vertices[index] -= centre
+	var normals := _field.cell_block_normals(start_column, end_column, start_row, end_row)
 	var indices := PackedInt32Array()
 	var columns_in_tile := end_column - start_column + 1
-	for row in range(start_row, end_row + 1):
-		for column in range(start_column, end_column + 1):
-			vertices.append(_field.cell_world_position(column, row) - centre)
-			normals.append(_field.normal_at_cell(column, row))
 	for row in range(end_row - start_row):
 		for column in range(end_column - start_column):
 			var top_left := row * columns_in_tile + column