feat(stdlib): Tiled P4 — objects, custom props/types, templates, opt-in spawn (#72)

- Object layers: all shapes (rectangle/ellipse/point/polygon/polyline/text) and
  custom properties parse and are queryable — Tiled.object_count / Tiled.object /
  Tiled.object_shape, and Tiled.prop / prop_int / prop_type over any object /
  tile / layer / map.
- Custom types: Tiled.load_types reads an objecttypes.xml project custom-type
  table so a class property resolves its default; enum values resolve as strings.
- Templates: Tiled.template reads a .tx/.tj, and Tiled.load merges each object's
  `template` reference under the instance's overrides (field inheritance).
- Opt-in spawning: Tiled.spawn / Tiled.spawn_layer map an object's class +
  properties onto Ludic components through the reflection ABI (Position from x/y,
  each property to a like-named field). Off by default — no gameplay coupling in
  the core load. Split into tiled_spawn.ludic, spliced only for a Tiled *game*
  (the world table exists only under has_ecs), so a plain map-reading tool never
  links against the reflection ABI.

Proven by library/tiled_p4.ludic (18 assertions) incl. the design's named
orthogonal-outside.tmx object shapes. x test: 95 passed.

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

docs/language/tiled/tiled-spawn_layer.md

@@ -0,0 +1,14 @@
+---
+id: tiled-spawn_layer
+name: Tiled.spawn_layer
+category: tiled
+kind: namespace-method
+tokens: Tiled.spawn_layer
+sig: Tiled.spawn_layer(map, layer: int, model: string) -> int
+tip: Spawn every object on a layer.
+order: 29
+ns: Tiled
+member: spawn_layer
+---
+
+Spawns every object on object <code>layer</code> as <code>model</code> (see <a href="tiled-spawn"><code>Tiled.spawn</code></a>); returns how many were spawned.