feat(stdlib): Tiled P0 — XML reader + base64 decode + gzip framing (#67)

The three parsing primitives the TMX path needs that were not already in
the tree (zlib inflate + the JSON reader already shipped):

- Xml.* — a minimal, deterministic pure-Ludic XML reader for the
  element/attribute/CDATA subset TMX/TSX/TX use, spliced on demand. Handles
  nested elements, single/double-quoted attributes, text + <![CDATA[…]]>,
  comments, the <?xml?> prolog and <!DOCTYPE>, the five predefined entities
  and numeric character references.
- Base64.* — standard base64 (RFC 4648) decode + a matching pure-Ludic
  encoder; the decoder ignores the whitespace Tiled wraps into <data>.
  Splicing Base64.* also pulls in inflate.ludic so a plain tool can run the
  full base64 -> zlib/gzip decode chain.
- z_gunzip — gzip framing (RFC 1952) over the existing DEFLATE inflater:
  skip the 10-byte header + optional fields, inflate the body, ignore the
  CRC32/ISIZE trailer.

Golden corpus vendored under assets/tiled-fixtures/ (mapeditor/tiled
examples, attributed, + hand-authored multi-encoding fixtures). New
example library/tiled_p0.ludic proves all three (21 assertions); docs
pages + inventory added so check-impl stays green.

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

assets/tiled-fixtures/README.md

@@ -0,0 +1,29 @@
+# Tiled golden fixtures
+
+The curated, version-pinned corpus for the Ludic Tiled reader (design record:
+[Design: Tiled maps](https://git.workshopsoft.io/workshopsoft/ludic/wiki/Design%2FTiled),
+issues #67–#74). No single Tiled file covers the format surface, so this is a
+subset of the official [`mapeditor/tiled`](https://github.com/mapeditor/tiled)
+`examples/` tree plus a few hand-authored files for the gaps the official
+examples miss.
+
+## Vendored from mapeditor/tiled (`examples/`)
+
+Fetched from `https://raw.githubusercontent.com/mapeditor/tiled/master/examples/`.
+Tiled's example assets carry their own licenses (see the upstream repo's
+per-folder `*.license`/README); vendored here with attribution for testing only.
+
+| File | Exercises |
+|---|---|
+| `desert.tmx` + `desert.tsx` | orthogonal, external `.tsx`, base64+zlib (P0/P1) |
+| `sewers.tmx` | orthogonal, base64+zlib, embedded tileset, opacity (P0) |
+| `orthogonal-outside.tmx` | object layers, shapes, custom properties (P4) |
+| `perspective_walls.tsx` | per-tile bool properties, `<tileoffset>` (P4) |
+| `isometric_grass_and_water.tmx` | isometric orientation, Wang set (P5) |
+| `hexagonal-mini.tmx` | hexagonal orientation (P5) |
+
+## Hand-authored (CC0 / public domain)
+
+| File | Exercises |
+|---|---|
+| `handmade.tmx` + `handmade.tsx` | a 4×4 orthogonal map whose four layers carry the **same** GID array in CSV, base64-uncompressed, base64+gzip and base64+zlib — so every encoding path must decode identically. The last tile is GID 1 with the horizontal-flip flag (`0x80000001`), forcing real GID decode. `handmade.tsx` also carries a `solid` bool property and a per-tile `<objectgroup>` collision shape for P2. |