Let a texture frame set where a sprite stands for Y sorting (#228)

A sprite sorted by Y stood on the bottom of its box. With a shadow or
empty rows under the feet that is too low, so a figure slipped in
front of a tree it should stand behind. The line belongs to the frame,
not the texture, because one texture can hold several sprite sheets.

- New field `sortLine` on a frame: whole image pixels up from the
  frame's bottom edge, 0 to the frame height. 0 is stored as nothing,
  so old projects stay the same. Shrinking a frame keeps the line
  inside it.
- `feetOf` in drawOrder.ts lifts a sprite by the same share of its
  box. Scene view, game view, play and picking all use it. In an
  animation the line of the frame shown right now counts.
- The texture tab draws the selected frame's line dashed in yellow
  with a knob left of the frame. At 0 the line shares the bottom edge
  with the nine-slice border and a resize handle, so the knob is
  tested first. A line inside the frame can be dragged directly,
  tested after the nine-slice lines.
- The inspector has a new "Sorting" section with a "Sort line"
  field. Dragging or typing is one undo step each.

Limit: frames of one walk cycle need the same value, set by hand.
Otherwise the figure changes its place in the order while walking.

Tests:
- ./check.sh: 160 files, 2356 tests, CHECK EXIT 0 (before 2350).
- 6 new unit tests in tests/viewport/sort-by-y.test.ts.
- e2e 37 and 39-43: ALL PASSED, 162 checks. New check 37b-ii drags
  the knob up 6 px and undoes it, plus the inspector row.
- Counter checks: if feetOf ignores the line, exactly the 3 order
  tests fail. If the knob cannot be grabbed, 37b-ii fails.
- The e2e sum in the README is 1380, counted, not yet seen in CI.

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

tools/e2e/37-nine-slice.mjs

@@ -109,6 +109,29 @@ export async function nineSlice(t) {
   );
   await redo();
   await redo();
+
+  // 37b-ii. The sort line (#228) sits on the bottom edge with the bottom border and the resize
+  //         handle, so it is dragged by its knob left of the frame: 6 pixels up, one undo step.
+  const sortLine = () =>
+    json(
+      `__gm.state.project.assets.find((a) => a.id === ${JSON.stringify(texture.id)})` +
+        `.frames.find((f) => f.id === 'ns-frame').sortLine ?? null`,
+    );
+  const knob = (await json('__gm.texture.drawInfo()')).sortKnob;
+  const knobTo = await json('__gm.texture.pointAt(0, 18)');
+  await drag(knob.x, knob.y, knob.x, knobTo.y);
+  await settle();
+  const lifted = await sortLine();
+  const afterLift = await frame();
+  await undo();
+  const dropped = await sortLine();
+  check(
+    'dragging the knob sets the sort line and leaves the frame and its borders, undo takes it away',
+    lifted === 6 && dropped === null && afterLift.h === 24 && afterLift.border?.bottom === 0,
+    JSON.stringify({ knob, lifted, dropped, afterLift }),
+  );
+  check('the sort line is a row of the frame inspector', await has('Sort line'));
+
   await mutate(`
     const f = __gm.state.project.assets.find((a) => a.id === ${JSON.stringify(texture.id)})
       .frames.find((f) => f.id === 'ns-frame');