Describe the terrain autotile kind

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

Development.md

@@ -14,7 +14,7 @@ npm run build        # writes dist/ and regenerates index.html
 ./build.sh           # the same under `set -o pipefail`, for callers that pipe the output
 php tools/make-user.php <name> <password> [admin|user]        # an account; every tool signs in (see Accounts)
 php tests/php/accounts.test.php                               # the API's account rules, no server needed (CI's gate job)
-node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1363 editor checks, screenshots in /tmp/gm-e2e-*.png
+node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1367 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

Tiles.md

@@ -5,15 +5,16 @@ same grass is used by several maps, the same way a texture is worn by several sp
 the asset was called *autotile*; a project still holding that type loses those assets on load
 (no migration, #137).
 
-A tile is one of three **kinds**:
+A tile is one of four **kinds**:
 
 | Kind | Block | Tiles it produces | What it does |
 |---|---|---|---|
 | **None (1 × 1)** | 1 tile | 1 | the picture as it is, repeated in every painted cell — the default |
 | **Floor (2 × 3)** | 2 tiles wide, 3 tall | 47 | an **autotile**: edges, convex corners and **notches** (inner corners) |
 | **Wall (2 × 2)** | 2 tiles wide, 2 tall | 16 | an **autotile**: edges and convex corners, no notches |
+| **Terrain (6 × 4)** | 6 tiles wide, 4 tall | 47 | an **autotile** in the layout of the common top-down sheets, see [below](#terrain) |
 
-A plain tile (*None*) ignores its neighbours. The two autotile kinds draw their own edges: paint a
+A plain tile (*None*) ignores its neighbours. The autotile kinds draw their own edges: paint a
 patch of grass onto a map and the borders towards everything else — the rounded corners, the
 straight edges, the notch where two edges meet — are picked per cell from one hand-drawn **block**.
 
@@ -37,6 +38,33 @@ in the sheet. The borders behave normally, left on the left and top on top. The
 RPG Maker MZ (`Tilemap.FLOOR_AUTOTILE_TABLE`), and the tests check the editor against that table
 row by row.
 
+### Terrain
+
+The terrain block is the layout many top-down tilesets use for ground, cliffs or stone patches:
+
+```
+ column   0           1-3                     4 | 5
+ row 0    strip top   top-left, top, top-right    notch bottom right | notch bottom left
+ row 1    strip mid   left, inside, right         notch top right    | notch top left
+ row 2    strip end   bottom-left, bottom, ...    notches TL + BR    | notches TR + BL
+ row 3    lone tile   strip left, middle, right   (unused)
+```
+
+- **Columns 0-3** hold one whole tile for each of the 16 ways the four sides can join: the
+  bordered 3x3, a vertical and a horizontal strip, and the lone tile.
+- **Columns 4-5** hold the notches. The 2x2 on top is a small hole seen from its four sides, so
+  each of its tiles has one notch. Row 2 has the two diagonal pairs. The two tiles in row 3 are
+  not used.
+- **A tile the sheet draws is taken whole.** A strip or a lone rock looks the way the artist drew
+  it, even where it differs from the 3x3.
+- **The rest is composed**: an edge with a notch, or notches next to each other. It is the tile of
+  its sides with the notch quarters put in. A notch also brings the quarter above or below it
+  from the same notch tile, when that quarter would be plain inside. That is what keeps a cliff's
+  notch whole: its wall reaches over the crossing into the quarter above. On flat ground that
+  quarter is plain inside anyway, so nothing changes.
+- The inside is **not** turned by half a tile as in a floor block. The 3x3 is an ordinary
+  nine-slice.
+
 **The tile size comes from the picture**: the kind says how many tiles wide a block is, so a 96 px
 wide floor block means 48 px tiles and 24 px quarters, and a 48 px square picture is a 48 px plain
 tile. A plain tile only has to be square — an odd side is fine; an autotile block must split evenly
@@ -54,7 +82,7 @@ asks for; a picture that does not fit is kept and named, never silently dropped.
   like any other picture — it has no frames, so it is always the whole canvas —, or
   [pixel art](Pixel-Art), whole or one of its frames. Cutting frames is
   the [Textures](Textures) tab, by hand or with **Slice…** where the sheet is a regular grid.
-- **Kind** — *None (1 × 1)*, *Floor (2 × 3)* or *Wall (2 × 2)*. Chosen, not guessed: a sheet whose rows alternate
+- **Kind** — *None (1 × 1)*, *Floor (2 × 3)*, *Wall (2 × 2)* or *Terrain (6 × 4)*. Chosen, not guessed: a sheet whose rows alternate
   between the two shapes (`Outside_A4` in RPG Maker) would be read wrong either way, and the choice
   is one click.
 - **Frames per second** and **Playback** (*Loop* or *Ping-pong*) — how the pictures run. One