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/editor/debug/sceneHooks.ts

@@ -101,6 +101,7 @@ export function sceneHooks({ state, editor }: HookContext) {
           name: layer.name,
           hidden: layer.hidden === true,
           z: layerZ(layer),
+          sortByY: layer.sortByY === true,
           cells: Object.keys(layer.cells).length,
         })) ?? null,
       /** What the scene view drew: meshes (one draw call each), cells and visible layers. */
@@ -164,6 +165,11 @@ export function sceneHooks({ state, editor }: HookContext) {
         const field = widgets && target && widgets.layers.valueField(paintedLayerId(state, target.asset));
         return field ? { ...centreOf(field), value: field.value } : null;
       },
+      /** The Sort by Y box of the painted layer (#228), with whether it is ticked. */
+      sortByYAt: () => {
+        const box = currentTilemapWidgets()?.sortByY;
+        return box ? { ...centreOf(box), checked: box.checked } : null;
+      },
       /** The duplicate mark on the painted layer's row. */
       duplicateLayerAt: () => {
         const widgets = currentTilemapWidgets();