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>

src/core/model/Asset.ts

@@ -109,6 +109,11 @@ export interface TextureFrame {
    * – its bordered tile and its notches. Absent is none; one of fewer than three corners collides as none.
    */
   collision?: Vec2[][];
+  /**
+   * Where a sprite wearing this frame stands when a layer sorts by Y (#228): whole image pixels up
+   * from the frame's bottom edge. Absent is 0, the bottom edge itself.
+   */
+  sortLine?: number;
 }
 
 /**

src/core/model/texture.ts

@@ -33,6 +33,8 @@ export interface TextureRef {
   sourceHeight: number;
   /** Set when the frame this ref names is sliced (#41); null means "draw it over the whole box". */
   slice: NineSlice | null;
+  /** The frame's sort line (#228) in image pixels up from its bottom; absent is 0. */
+  sortLine?: number;
 }
 
 /**
@@ -119,6 +121,7 @@ function generatedRef(asset: Asset, project: Project, rect: FrameRect | null = n
 
 function refFor(asset: TextureAsset, frame: TextureFrame | null): TextureRef | null {
   if (!isTextureFile(asset.file)) return null;
+  const sortLine = frame ? frameSortLine(frame) : 0;
   return {
     file: asset.file,
     smooth: textOf(asset.props.filter ?? 'linear') !== 'nearest',
@@ -126,9 +129,28 @@ function refFor(asset: TextureAsset, frame: TextureFrame | null): TextureRef | n
     sourceWidth: asset.width,
     sourceHeight: asset.height,
     slice: frame ? nineSliceOf(frame, sliceScaleOf(asset)) : null,
+    ...(sortLine > 0 ? { sortLine } : {}),
   };
 }
 
+/** Whole pixels from the frame's bottom edge up to its top edge at most. */
+export function normalizeSortLine(value: number, h: number): number {
+  const line = Math.round(Number.isFinite(value) ? value : 0);
+  return Math.max(0, Math.min(Math.floor(h), line));
+}
+
+/** Read against the frame as it is now, so a frame resized smaller keeps its line inside it. */
+export function frameSortLine(frame: TextureFrame): number {
+  return normalizeSortLine(frame.sortLine ?? 0, frame.h);
+}
+
+/** 0 removes the key, so a frame nobody gave a line saves as it did before. */
+export function setFrameSortLine(frame: TextureFrame, value: number | undefined): void {
+  const line = normalizeSortLine(value ?? 0, frame.h);
+  if (line > 0) frame.sortLine = line;
+  else delete frame.sortLine;
+}
+
 const NO_BORDERS: FrameBorders = { left: 0, top: 0, right: 0, bottom: 0 };
 export const DEFAULT_FILL: FrameFill = 'repeat';