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/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