Describe deleting a painted tile on Tiles and Tilemaps (#268)

- Tiles: deleting a tile empties its cells, the dialog counts them,
  and undo brings both back. A cell naming a missing tile is empty.
- Tilemaps: Fill and "Tile at" treat such a cell as empty, and a
  deleted tile in hand no longer paints.
- Development: the e2e sum is 1590.

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

Development.md

@@ -18,7 +18,7 @@ php tests/php/accounts.test.php                               # the API's accoun
                                                               #   has and the web server's container has not (SKIP there)
 php tests/php/snapshots.test.php                              # the snapshot store (#231), the same way
 php tests/php/releases.test.php                               # the release store (#266), the same way
-node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1586 editor checks, screenshots in /tmp/gm-e2e-*.png
+node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1590 editor checks, screenshots in /tmp/gm-e2e-*.png
 GM_STEPS=20 node tools/e2e.mjs                                # or 1-12, or 6,20 — a subset while working on one step
 ./e2e.sh start [GM_STEPS]                                     # the same run detached, see below
 ./e2e.sh status                                               # live progress, or the last run's result

Tilemaps.md

@@ -77,7 +77,8 @@ the Columns, Rows and Tile size rows.
 - **Fill** (#133) fills, from the cell clicked, every cell of the active layer that holds the same
   tile — or is empty, if that one is — and is reached through the four edge neighbours. Diagonals do
   not count, so a fill does not leak through the gap between two corners. The grid is the border.
-  Filling a cell with the tile it already holds does nothing.
+  Filling a cell with the tile it already holds does nothing. A cell naming a tile the project
+  does not have counts as empty (see [Tiles](Tiles#where-they-are-placed)).
 - **Pick** takes the tile of the cell clicked on the **active layer** and hands it straight to the
   painting tool used before Pick (Brush, Rect or Fill; from Select or Erase it is the Brush). An
   empty cell says so in the console and changes nothing.
@@ -98,7 +99,8 @@ Under **TILES** stands every tile of the project as a grid of pictures with its
 (#139), so you see what you paint with. A floor or wall tile shows its lone tile — the four
 convex corners, the one RPG Maker shows as well — and an animated tile its first picture. A tile
 without a usable picture shows a **?** and can still be picked. Picking one reaches for the brush by
-itself. Without one chosen, painting says so instead of quietly doing nothing.
+itself. Without one chosen, or with one deleted since, painting says so instead of quietly doing
+nothing.
 
 ## Layers
 
@@ -182,8 +184,8 @@ A behavior reads and changes the cells of a placed map while the game runs (#135
 it, each taking the placed map as its `tilemap` input:
 
 - **`Tile at`** gives the tile in a cell (`tile`, empty for none) and `empty`. It reads the moment
-  it is asked, so a `Set tile` earlier in the same flow is already seen. A cell outside the grid or
-  on a layer the map does not have reads as empty.
+  it is asked, so a `Set tile` earlier in the same flow is already seen. A cell outside the grid,
+  on a layer the map does not have or naming a tile the project does not have reads as empty.
 - **`Set tile`** puts a tile into a cell, or empties it when no tile is given. `done` says whether
   it could: not on something that is no tilemap, not on a missing layer, not outside the grid, and
   not with an asset that is no tile.

Tiles.md

@@ -139,3 +139,12 @@ Kind, rate, playback, the solid switch, the collision scale and the sort offset
 A tile is never placed by itself: it is laid down by a [tilemap](Tilemaps), which is the asset
 that carries a grid of cells and is placed in a scene. Two different tiles do not join each other
 there — each autotile closes against everything else, as the RPG-Maker sheets are drawn.
+
+**Deleting a tile** that is painted on a map empties its cells in the same step (#268). The delete
+dialog says how many cells on how many tilemaps, and Ctrl+Z brings the tile and its cells back
+together. A brush still holding the deleted tile paints nothing and asks for a tile.
+
+A cell can still name a tile the project does not have, in an older file or one edited by hand.
+It is empty for everything that looks at the map: it is not drawn, it does not block, `Tile at`
+reads it as empty and **Fill** fills across it. Only **Pick** says what it is. The file keeps it
+until the cell is painted over or erased.