Add a terrain autotile kind for 6x4 sheets like TD Plains

Sheets such as "TD - Tileset - Plains" draw an autotile as a 6 x 4
block: a 3 x 3 nine-slice, a vertical and a horizontal strip and a
single tile on the left, the inner corners on the right. The floor
kind only knew the RPG Maker 2 x 3 layout, so these sheets could not
be used as autotiles at all.

- New TileKind "terrain" with 16 px tiles in a 6 x 4 block. All 47
  neighbour combinations are resolved per quarter, so the renderer,
  the collision and the palette stay untouched.
- Whole tiles are used where the sheet draws them: the 16 side cases,
  the four single notches and the two diagonal pairs. Hand-drawn
  strips and single blocks therefore look as drawn.
- Only the missing cases are composed, for example an edge with a
  notch or two notches side by side. They take the side tile and put
  the notch quarters in.
- A notch also takes the quarter above or below from its notch tile
  when fill would stand there. Tall cliff notches keep their wall.
  On ground and stones fill meets fill, so nothing changes there.
- The label rule for floor and terrain now runs through one notch
  function. Floor still passes the MZ table test.

Not touched: the tile board only refits on open and on Fit. Switching
an open tile to a kind with another tile size leaves it small until
Fit is pressed. That was the case before.

Tests:
- ./check.sh: 156 files, 2290 tests, CHECK EXIT 0 (before 2283).
- e2e 39,40: ALL PASSED, 90 checks (before 86). 39j checks 47 tiles,
  pixels of single, notch, pair and composed tiles, and undo to floor.
- Counter-checks: without whole notch and pair tiles one unit test
  and one e2e check fail. Without the taken quarter one of each
  fails. Originals restored by cmp and rebuilt.
- Check sum: 1363 measured by CI plus 4 is 1367, computed only.

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

src/core/properties/PropertySchema.ts

@@ -368,6 +368,7 @@ const ASSET_SCHEMAS: Record<AssetType, PropertySchema> = {
         { value: 'none', label: 'None (1 × 1)' },
         { value: 'floor', label: 'Floor (2 × 3)' },
         { value: 'wall', label: 'Wall (2 × 2)' },
+        { value: 'terrain', label: 'Terrain (6 × 4)' },
       ],
     },
     { key: TILE_FPS_KEY, label: 'Frames per second', kind: 'number', default: 4, min: 0.1, max: 120 },