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>

docs/language/xml/xml-parse.md

@@ -0,0 +1,14 @@
+---
+id: xml-parse
+name: Xml.parse
+category: xml
+kind: namespace-method
+tokens: Xml.parse
+sig: Xml.parse(text: string) -> xml
+tip: Parse XML text into an element tree.
+order: 1
+ns: Xml
+member: parse
+---
+
+Parses an XML document and returns its root element (or an empty node if there is none). Handles nested elements, single- and double-quoted attributes, text/<code>&lt;![CDATA[…]]&gt;</code> content, comments, the <code>&lt;?xml?&gt;</code> prolog and <code>&lt;!DOCTYPE&gt;</code>. Best-effort, not validating.