Empty a deleted tile's cells on every tilemap (#268)

Deleting a tile left the cells painted with it in the tilemaps. They
drew nothing, blocked nothing and did not show in the delete dialog,
but still read as a tile for "Tile at" and stopped a fill.

- DeleteAssetCommand empties the cells of a deleted tile on every
  layer of every tilemap in the same step. Undo puts the tile and its
  cells back together.
- The delete dialog counts them, for example "3 cell(s) on 1
  tilemap(s) are painted with it and become empty." (cellsPainted).
- presentTile treats a cell naming a missing tile as empty. "Tile at"
  (World.tileAt) and floodFill use it, so cells already in existing
  files read and fill as empty. There is no migration.
- The brush, the rectangle and the fill check the tile in hand. A
  tile deleted after it was picked no longer paints new invisible
  cells, the editor says "Choose a tile to paint with." instead.
- Unchanged on purpose: the picker still reports such a cell as
  naming a tile that no longer exists, and erasing removes it.

Tests:
- ./check.sh: 195 files, 2823 tests, CHECK EXIT 0 (+6).
- GM_STEPS=39,40: ALL PASSED, 100 checks (4 new in 40t).
- Full e2e sum is now 1590 (1586 confirmed by CI on round 8, plus 4).
- Counter-checks, all fail as expected: 5 in unit tests (delete
  without emptying, undo without cells, fill and "Tile at" without
  presentTile, wrong map count) and 3 in the browser (dialog without
  count, delete without emptying, brush without the check).

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

src/core/model/tilemap.ts

@@ -69,6 +69,21 @@ export function cellAt(layer: TilemapLayer, x: number, y: number): string {
   return layer.cells[cellKey(x, y)] ?? '';
 }
 
+/**
+ * The tile a cell holds as far as anything but the file is concerned: `''` for an empty cell and for
+ * one naming a tile the project does not have (#268). Such a cell draws and blocks nothing, so it
+ * reads and fills as empty too.
+ */
+export function presentTile(project: Project, tileId: string): string {
+  return tileId !== '' && findAssetOf(project, tileId, 'tile') ? tileId : '';
+}
+
+/** The cells without those naming `tileId`, or null when none does. */
+export function cellsWithout(cells: Record<string, string>, tileId: string): Record<string, string> | null {
+  if (!Object.values(cells).includes(tileId)) return null;
+  return Object.fromEntries(Object.entries(cells).filter(([, id]) => id !== tileId));
+}
+
 export function isLayerVisible(layer: TilemapLayer): boolean {
   return layer.hidden !== true;
 }
@@ -122,8 +137,8 @@ export interface TileDraw {
 
 /**
  * Every cell of one layer that is inside the grid and names a tile that exists, in reading order
- * so the result is the same list twice. Cells naming a deleted tile are skipped rather than
- * dropped from the model: an undo brings the tile, and with it the drawing, back.
+ * so the result is the same list twice. A cell naming a tile the project does not have is skipped
+ * (see `presentTile`).
  */
 export function layerTiles(project: Project, props: PropertyBag, layer: TilemapLayer): readonly TileDraw[] {
   const kinds = new Map<string, TileKind>();
@@ -255,11 +270,24 @@ export function cellsInRect(props: PropertyBag, a: Cell, b: Cell): Cell[] {
  * The cells a fill from `start` changes (#133): every cell reached through its four edge
  * neighbours that holds what `start` holds – empty included – clipped to the grid. Diagonals do not
  * count, or a fill would leak through the gap between two corners. Filling with the tile already
- * there changes nothing, so that returns no cells.
+ * there changes nothing, so that returns no cells. A cell naming a missing tile is empty here.
  */
-export function floodFill(props: PropertyBag, layer: TilemapLayer, start: Cell, tileId: string): Cell[] {
+export function floodFill(
+  project: Project,
+  props: PropertyBag,
+  layer: TilemapLayer,
+  start: Cell,
+  tileId: string,
+): Cell[] {
   if (!inGrid(props, start.x, start.y)) return [];
-  const from = cellAt(layer, start.x, start.y);
+  const present = new Map<string, string>();
+  const tileAt = (x: number, y: number): string => {
+    const id = cellAt(layer, x, y);
+    let shown = present.get(id);
+    if (shown === undefined) present.set(id, (shown = presentTile(project, id)));
+    return shown;
+  };
+  const from = tileAt(start.x, start.y);
   if (from === tileId) return [];
   // The list is its own queue: everything in it is filled, and it is walked while it grows.
   const cells = [{ x: start.x, y: start.y }];
@@ -275,7 +303,7 @@ export function floodFill(props: PropertyBag, layer: TilemapLayer, start: Cell,
       const x = cell.x + dx;
       const y = cell.y + dy;
       const key = cellKey(x, y);
-      if (seen.has(key) || !inGrid(props, x, y) || cellAt(layer, x, y) !== from) continue;
+      if (seen.has(key) || !inGrid(props, x, y) || tileAt(x, y) !== from) continue;
       seen.add(key);
       cells.push({ x, y });
     }

src/core/model/usages.ts

@@ -68,6 +68,22 @@ export function tilesUsing(project: Project, ids: readonly string[]): TileAsset[
   );
 }
 
+/** The cells a tile is painted on, and on how many tilemaps – the cells are the maps' own data. */
+export function cellsPainted(project: Project, tileId: string): { tilemaps: number; cells: number } {
+  let tilemaps = 0;
+  let cells = 0;
+  for (const asset of project.assets) {
+    if (asset.type !== 'tilemap') continue;
+    const here = asset.layers.reduce(
+      (sum, layer) => sum + Object.values(layer.cells).filter((id) => id === tileId).length,
+      0,
+    );
+    if (here > 0) tilemaps++;
+    cells += here;
+  }
+  return { tilemaps, cells };
+}
+
 /** The animations keying any of these ids – intrinsic data, which `findPropertyUsages` does not see. */
 export function animationsUsing(project: Project, ids: readonly string[]): AnimationAsset[] {
   const wanted = new Set(ids);