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>

tools/e2e/40-tilemap.mjs

@@ -571,6 +571,29 @@ export async function tilemap(t) {
     JSON.stringify({ withZ, orderZ, orderBack, zBack }),
   );
 
+  // 40p-iii. Sort by Y (#228) draws the painted layer row by row: its one cell (4,0) becomes the
+  //          slot of row 0 in place of the whole layer's. Undone again for the blocks below.
+  const sortBox = await inPanel(t, '__gm.tilemap.sortByYAt()');
+  if (sortBox) await click(sortBox.x, sortBox.y);
+  await settle();
+  const sortedLayer = (await json(`__gm.tilemap.layers(${JSON.stringify(made.id)})`))[1];
+  const orderSorted = await json('__gm.drawOrder()');
+  const boxSorted = await json('__gm.tilemap.sortByYAt()');
+  await undo();
+  await settle();
+  const orderUnsorted = await json('__gm.drawOrder()');
+  check(
+    'Sort by Y draws the painted layer as one slot per row, and undo makes it one piece again',
+    sortBox?.checked === false &&
+      sortedLayer.sortByY === true &&
+      boxSorted?.checked === true &&
+      orderSorted.includes(`${upperSlot}#0`) &&
+      !orderSorted.includes(upperSlot) &&
+      orderUnsorted.includes(upperSlot) &&
+      !orderUnsorted.includes(`${upperSlot}#0`),
+    JSON.stringify({ sortBox, sortedLayer, boxSorted, orderSorted, orderUnsorted }),
+  );
+
   // 40p-ii. The duplicate mark copies the painted layer right above it, cells and all, and paints
   //         on the copy. Undone again: the blocks below count on two layers.
   const copyMark = await inPanel(t, '__gm.tilemap.duplicateLayerAt()');