Tilemaps: a painted cell works out only what is around it (#348)

- A fourth round on painting: the keys a writer changed, patching the
  changed cells and their neighbours, merging row by row, tiles sorted
  by id.
- The measurements on 60x60 and 120x120, and that the meshes of a
  layer still grow with the map.

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

Tilemaps.md

@@ -374,8 +374,27 @@ Measured with a brush stroke on a copy of a project with a 60x60 map on three la
 objects (Chrome, slow machine): 20 to 39 fps before, 47.5 after, against 55 to 57 fps without
 painting. The stroke drew 2012 labels again before and none after, and the scene 1.32 times a
 frame before and once after. One painted cell cost the scene view 28 ms headless before and 16 ms
-after, on the same map doubled to 120x120 109 ms before and 49 ms after. That last part still grows
+after, on the same map doubled to 120x120 109 ms before and 49 ms after.
-with the map: a painted cell works out the tiles and blocking cells of the whole map again.
+
+A fourth round (#348) made a painted cell cost what is around it, not the whole map:
+
+- `cellsChanged` takes the keys a writer changed (the brush, the paint command and its undo), and
+  each new version of the cells keeps them (`cellsChangedSince`, the last 16 versions). A writer
+  that names none costs one walk over the cells, and a replaced object starts afresh.
+- The cell index, the tiles of a layer and the blocking cells patch only the changed cells and their
+  eight neighbours: nothing else can join or block otherwise. Both lists are in cell order now, so a
+  patch is one walk that keeps every other entry, the same object as before.
+- The rectangles are merged row by row. A strip never spans two rows, so a patch joins again only
+  the rows around it and stacks the rest as they were. That is one row further than the patched
+  cells, since an autotile turning into a box cuts the box below it otherwise.
+- The tiles a map names come sorted by id, so the same content gives the same order whichever way it
+  was painted. That is also the order of two trigger tiles on one cell.
+
+Measured with the same map: one painted cell costs the scene's item list 3.5 ms on 60x60 and 4.0 ms
+on 120x120, against 3.4 ms without painting (headless, before 16.8 and 54.6 ms). A brush stroke on
+the map doubled to 120x120 runs at 42 fps instead of 28, p95 of a frame 52 instead of 92 ms. What
+still grows with the map is the mesh: a painted cell builds the geometry of every tile on that layer
+again, since the layer's tiles come as a new list and are grouped by tile afresh.
 
 A frame still costs what it touches: an object standing **inside** many outlined cells is tested
 against each of them, so an outline of a hundred corners around every cell of a lake is dearer than