Let a tilemap layer sort its rows by Y with the objects (#228)

A tilemap layer was one piece in the draw order, so a figure stood
either in front of a whole layer of trees or behind all of it. For a
top-down game the trees in front of the figure's feet have to cover
it and the ones behind must not.

- A new "Sort by Y" box under the layer list sets `sortByY` on the
  painted layer. Every change is its own undo step.
- Such a layer comes as one draw slot per row. On its `z`, the rows
  and all objects on that `z` are sorted by where they stand: the
  bottom of an object's box, the bottom of a row. Ties keep the
  hierarchy order, and other `z` values are untouched.
- A layer drawn as one piece and the map's own node stay behind the
  sorted ones on the same `z`, since neither has a single place to
  stand. Ground belongs on a lower `z` or stays unsorted.
- A new tile property "Sort offset" (whole rows, -16 to 16) lets a
  crown sort with the trunk below it.
- `TilemapField` draws a container per row for such a layer, and
  picking takes the row into account, so a click picks what is drawn
  in front. Scene view, game view and play share the same rule.

Limits: on a rotated map "down" is world Y. A layer sorted by Y costs
about one draw call per row and tile instead of one per tile.

Tests:
- ./check.sh: 160 files, 2350 tests, CHECK EXIT 0 (before 2340).
- 10 new unit tests in tests/viewport/sort-by-y.test.ts.
- e2e 39-43: ALL PASSED, 148 checks. New check 40p-iii ticks the box
  and finds the slot of row 0, undo brings back the whole layer.
- Counter checks: without the Y sort 5 unit tests fail, with the
  offset ignored 5, with no row in picking 1. If TilemapField draws
  no rows, exactly 40p-iii fails in e2e.
- The e2e sum in the README is 1378, counted, not yet seen in CI.

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

src/core/commands/tilemapCommands.ts

@@ -309,6 +309,33 @@ function setLayerZ(layer: TilemapLayer, z: number): void {
   else layer.z = z;
 }
 
+/** Whether a layer is drawn row by row and sorted in by Y (#228); off is stored as nothing. */
+export class SetTilemapLayerSortByYCommand implements Command {
+  readonly label: string;
+
+  constructor(
+    private readonly assetId: string,
+    private readonly layerId: string,
+    private readonly sortByY: boolean,
+  ) {
+    this.label = sortByY ? 'Sort layer by Y' : 'Stop sorting layer by Y';
+  }
+
+  execute(project: Project): void {
+    this.apply(project, this.sortByY);
+  }
+
+  undo(project: Project): void {
+    this.apply(project, !this.sortByY);
+  }
+
+  private apply(project: Project, sortByY: boolean): void {
+    const layer = requireTilemapLayer(project, this.assetId, this.layerId);
+    if (sortByY) layer.sortByY = true;
+    else delete layer.sortByY;
+  }
+}
+
 /** A hidden layer keeps its cells and is left out of the drawing, in the editor and in the game alike. */
 export class SetTilemapLayerHiddenCommand implements Command {
   readonly label: string;