Painting over identical tiles no longer adds an empty undo step (#212)

A rectangle drawn over cells that already hold exactly that tile
pushed a command that changed nothing. Undo then seemed to do nothing
and the project was marked as modified. PaintTilesCommand.changes()
was meant to prevent that, but nothing called it since #128.

- New paintTiles() in tilemapCommands.ts returns the command for a
  finished stroke or fill, or null when no cell changes. It is the
  first caller of PaintTilesCommand.changes().
- All three paint paths in the Scene View go through it: brush and
  eraser, rectangle (both in endStroke) and flood fill (fillFrom).
- Flood fill was not affected, because floodFill already returns []
  for the same tile. The guard is there anyway, as the issue asked.
- A rectangle that changes even one cell stays a normal undo step.

Tests:
- ./check.sh: 145 files, 2190 tests, CHECK EXIT 0 (before 2187).
- 3 new unit tests in tests/core/tilemap.test.ts: same rectangle
  twice gives no command, a wider rectangle still does, a fill that
  finds nothing gives no command.
- 1 new e2e check in 40d: the same rectangle again, then one undo
  must take 9 cells to 0. A redo restores them for later blocks.
- GM_STEPS=39,40 on the final build: ALL PASSED, 81 checks.
- Counter-probe with changes(...) || true: exactly the two unit
  tests fail, and in the browser exactly the new check fails with
  {"afterUndo":9}. Original restored via cmp (IDENTICAL), rebuilt.
- Editor suite check total: CI measured 1312 at 0e85856, new total
  1313 (calculated) in README and wiki.

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

src/core/commands/tilemapCommands.ts

@@ -82,6 +82,21 @@ export class PaintTilesCommand implements Command {
   }
 }
 
+/**
+ * The command for a finished stroke or fill, or null when it would change no cell: such a step
+ * would undo nothing and still count as an edit of the project (#212).
+ */
+export function paintTiles(
+  layer: TilemapLayer,
+  assetId: string,
+  edits: readonly CellEdit[],
+  label?: string,
+): PaintTilesCommand | null {
+  return PaintTilesCommand.changes(layer, edits)
+    ? new PaintTilesCommand(assetId, layer.id, edits, label)
+    : null;
+}
+
 /**
  * Where a placed tilemap's origin goes when `edge` gains `count` cells, starting from `from`, so the
  * painted cells stay where they are in the scene (#136). The shift is worked out in the tilemap's