feat(stdlib): Tiled P0.5 — TMX/TSX reader over the XML reader (#68)

Tiled.read / Tiled.read_tsx (runtime/native/tiled.ludic): map the native
XML formats (TMX/TSX/TX) onto the SAME intermediate the JSON path produces
— a Value tree in Tiled's JSON schema — so one format-independent core
(P1) consumes either reader.

- tmx_to_value walks a <map> into {orientation, width/height, tilewidth/
  height, tilesets[], layers[]}; every tile layer's <data> is decoded to a
  dense GID int list (CSV split, or base64 -> zlib/gzip inflate ->
  little-endian u32s), so a CSV .tmx and a base64 .tmj of the same map read
  structurally identically.
- External tilesets keep the {firstgid, source} reference (as TMJ does);
  embedded tilesets and standalone .tsx (tsx_to_value) inline full geometry
  + per-tile metadata (animation frames, collision objectgroup, class,
  properties) for later phases.
- Object layers, shapes (rect/ellipse/point/polygon/polyline/text),
  image/group layers and custom properties are parsed into the tree now so
  P2/P4/P5 just read it.
- tmj_normalize decodes base64 `data` strings in a parsed .tmj so the JSON
  path matches the XML path.

Proven by library/tiled_p05.ludic (30 assertions) incl. the in-repo Kenney
sampleMap.tmx + external sampleSheet.tsx and TMX-vs-TMJ structural
identity. Docs + inventory added; x test: 91 passed.

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

assets/tiled-fixtures/handmade.tmj

@@ -0,0 +1,65 @@
+{
+ "type": "map",
+ "version": "1.10",
+ "tiledversion": "1.10.2",
+ "orientation": "orthogonal",
+ "renderorder": "right-down",
+ "width": 4,
+ "height": 4,
+ "tilewidth": 16,
+ "tileheight": 16,
+ "infinite": false,
+ "nextlayerid": 3,
+ "nextobjectid": 1,
+ "tilesets": [
+  {
+   "firstgid": 1,
+   "source": "handmade.tsx"
+  }
+ ],
+ "layers": [
+  {
+   "type": "tilelayer",
+   "id": 1,
+   "name": "csv",
+   "width": 4,
+   "height": 4,
+   "x": 0,
+   "y": 0,
+   "opacity": 1,
+   "visible": true,
+   "data": [
+    1,
+    2,
+    3,
+    4,
+    5,
+    6,
+    7,
+    8,
+    9,
+    10,
+    11,
+    12,
+    13,
+    14,
+    15,
+    2147483649
+   ]
+  },
+  {
+   "type": "tilelayer",
+   "id": 2,
+   "name": "zlib",
+   "width": 4,
+   "height": 4,
+   "x": 0,
+   "y": 0,
+   "opacity": 1,
+   "visible": true,
+   "encoding": "base64",
+   "compression": "zlib",
+   "data": "eJwNw4cNACAMBLEPvYaVGT1nySYpMbOwsrFzcHJx8/DS+WjSDw1EAPo="
+  }
+ ]
+}