feat(engine): Tiled P5 — image/group layers, iso/hex/staggered coords, Wang (#73)

- Image layers: <imagelayer> renders with parallax + optional repeatx/repeaty
  tiling across the view; the image loads relative to the map file. Group layers
  flatten at build (children render in file order); layer offset/opacity/tint are
  queryable (Tiled.layer_kind / layer_offsetx / layer_offsety / layer_opacity /
  layer_tint), a group's tint applying recursively.
- Orientation transforms: Tiled.cell_x / Tiled.cell_y compute a tile cell's
  screen position for orthogonal, isometric ((x-y)*tw/2, (x+y)*th/2), staggered,
  and hexagonal (rows step by (tileh+hexsidelength)/2, alternate rows shoved per
  staggerindex) — the tile draw now places cells through them.
- Wang: exported Wang-set GIDs are ordinary GIDs and resolve through the standard
  GID resolver; the terrain-corner authoring concept is editor-side (design cut).

Proven by library/tiled_p5.ludic (14 assertions) over the real
isometric_grass_and_water.tmx (iso + Wang) and hexagonal-mini.tmx, plus a
hand-authored image+group fixture. x test: 96 passed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/language/tiled/tiled-layer_offsety.md

@@ -0,0 +1,14 @@
+---
+id: tiled-layer_offsety
+name: Tiled.layer_offsety
+category: tiled
+kind: namespace-method
+tokens: Tiled.layer_offsety
+sig: Tiled.layer_offsety(map, layer: int) -> int
+tip: A layer's vertical offset.
+order: 36
+ns: Tiled
+member: layer_offsety
+---
+
+Returns a layer's vertical draw offset in pixels (0 when unset).