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>

selfhost/backend/emit_call.ludic

@@ -586,6 +586,27 @@ function emit_ns_call(ns: pointer, meth: pointer, e: Node) -> Val {
     if (meth == "encode")  { bare = "json_encode";   push(labels, "value") }
     if (meth == "parse")   { bare = "json_parse";    push(labels, "text") }
   }
+  # Xml.* — the minimal XML reader (runtime/native/xml.ludic, spliced on demand).
+  # parse returns an Xml node; the accessors read tag/text/attributes/children.
+  if (ns == "Xml") {
+    if (meth == "parse")       { bare = "xml_parse";       push(labels, "text") }
+    if (meth == "tag")         { bare = "xml_tag";         push(labels, "node") }
+    if (meth == "text")        { bare = "xml_text";        push(labels, "node") }
+    if (meth == "attr")        { bare = "xml_attr";        push(labels, "node"); push(labels, "key") }
+    if (meth == "attr_int")    { bare = "xml_attr_int";    push(labels, "node"); push(labels, "key"); push(labels, "dflt") }
+    if (meth == "has")         { bare = "xml_has";         push(labels, "node"); push(labels, "key") }
+    if (meth == "attr_count")  { bare = "xml_attr_count";  push(labels, "node") }
+    if (meth == "child_count") { bare = "xml_child_count"; push(labels, "node") }
+    if (meth == "child")       { bare = "xml_child";       push(labels, "node"); push(labels, "index") }
+    if (meth == "find")        { bare = "xml_find";        push(labels, "node"); push(labels, "tag") }
+    if (meth == "count")       { bare = "xml_count";       push(labels, "node"); push(labels, "tag") }
+  }
+  # Base64.* — standard base64 codec (runtime/native/base64.ludic, spliced on
+  # demand). decode/encode round-trip through NUL-terminated strings.
+  if (ns == "Base64") {
+    if (meth == "decode")  { bare = "base64_decode";  push(labels, "src") }
+    if (meth == "encode")  { bare = "base64_encode";  push(labels, "src") }
+  }
   # #62: a package-provided namespace (declared with @Namespace(Foo)) that none
   # of the hardcoded core blocks matched — alias Foo.method to the bare function
   # foo_method (positional args), the same generic path the core aliases use.