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>

tools/docgen/inventory.json

@@ -681,5 +681,9 @@
   "base64": [
     "base64-decode",
     "base64-encode"
+  ],
+  "tiled": [
+    "tiled-read",
+    "tiled-read_tsx"
   ]
 }