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>

README.md

@@ -43,7 +43,7 @@ npm run build        # writes dist/ and regenerates index.html; the folder is wh
 The browser suite and the three example games need a machine with a GPU:
 
 ```bash
-node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1312 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1313 checks (GM_STEPS=20 for one step)
                                                    #   every tool signs in first – see the Accounts page
 node tools/seed-jumprun.mjs && node tools/smoke-jumprun.mjs
 node tools/seed-tetris.mjs && node tools/smoke-tetris.mjs