Work out only the cells around a painted one on a tilemap (#348)

A painted cell worked out the tiles and blocking cells of the whole
map again, so painting got slower the bigger the map. One cell cost
16.8 ms on 60x60 and 54.6 ms on 120x120. Now it costs what is around
it: 3.5 ms and 4.0 ms, against 3.4 ms without painting.

- The brush, the paint command and its undo name the keys they
  changed. Each new version of the cells keeps them for the last 16
  versions (cellsChangedSince).
- The cell index, the tiles of a layer and the blocking cells patch
  only the changed cells and their eight neighbours. Everything else
  stays the same object as before.
- Both lists are in cell order now, so a patch is one walk that keeps
  every other entry.
- The rectangles are merged row by row. A patch joins again only the
  rows around it, one row further than the changed cells, and stacks
  the rest as they were.
- A writer that names no keys costs one walk over the cells. A
  replaced cell object (load, resize, edits at run time) is worked out
  in full as before.
- The tiles a map names come sorted by id. Two trigger tiles on one
  cell now keep that order, whichever way they were painted.
- resolveTilemap walked all tiles every frame, even without painting.
  It now does so only when the list changed.

Found on the way: deleting an outlined autotile can turn the one below
into a full box, which cuts a scaled box two rows further down. Merging
only the patched rows missed that. It is a test of its own now.

A brush stroke on the map doubled to 120x120 runs at 42 fps instead of
28, p95 of a frame 52 ms instead of 92 ms. What still grows with the
map is the mesh of a layer, which is built again on every paint.

Tests:
- tests/core/tilemap-patch.test.ts (new, 7): 400 random steps on two
  layers with every kind of tile, each compared with a fresh full run
- Unit, tilemap and viewport: 1284 green in 57 files
- e2e 39 + 40 (atx): ALL PASSED, 107 checks
- Gate (atx): 312 files, 4831 tests

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

src/editor/viewport/tilemapPainting.ts

@@ -200,7 +200,7 @@ export class TilemapPainter {
     const stroke = this.stroke;
     const layer = stroke && this.strokeLayer(stroke);
     if (!stroke || !layer) return;
-    let changed = false;
+    const changed: string[] = [];
     for (const at of cells) {
       if (!inGrid(item.props, at.x, at.y)) continue;
       const key = cellKey(at.x, at.y);
@@ -209,10 +209,10 @@ export class TilemapPainter {
       stroke.edits.push({ x: at.x, y: at.y, tileId: stroke.tileId });
       if (stroke.tileId) layer.cells[key] = stroke.tileId;
       else delete layer.cells[key];
-      changed = true;
+      changed.push(key);
     }
-    if (!changed) return;
-    cellsChanged(layer.cells);
+    if (changed.length === 0) return;
+    cellsChanged(layer.cells, changed);
     this.perFrame.request();
   }
 
@@ -229,7 +229,7 @@ export class TilemapPainter {
         if (value) layer.cells[key] = value;
         else delete layer.cells[key];
       }
-      cellsChanged(layer.cells);
+      cellsChanged(layer.cells, stroke.previous.keys());
     }
     const item = this.host.items().find((i) => i.id === stroke.objectId);
     const edits =