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>

assets/tiled-fixtures/img_group.tmx

@@ -0,0 +1,18 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- Hand-authored P5 fixture: image + group layers (issue #73). CC0. -->
+<map version="1.10" tiledversion="1.10.2" orientation="orthogonal" renderorder="right-down" width="4" height="4" tilewidth="16" tileheight="16" infinite="0" nextlayerid="4" nextobjectid="1">
+ <tileset firstgid="1" source="anim_tiles.tsx"/>
+ <imagelayer id="1" name="bg" offsetx="8" offsety="4" repeatx="1">
+  <image source="../kenney/tiny-dungeon/Tilemap/tilemap.png" width="203" height="186"/>
+ </imagelayer>
+ <group id="2" name="grp" offsetx="16" opacity="0.5" tintcolor="#ff0000">
+  <layer id="3" name="inner" width="4" height="4">
+   <data encoding="csv">
+2,0,0,0,
+0,0,0,0,
+0,0,0,0,
+0,0,0,0
+</data>
+  </layer>
+ </group>
+</map>