feat(draw): drawing-card document envelope model + parser (T-318)

DrawingCardDoc + parseDrawingCardDoc read the `clide draw` JSON: card
label/description, primitive svg/svgPath, or a template name plus its
fields (for a handler to lower to SVG). Tolerant — a non-object payload
is null, blank strings are absent, never throws. Flutter-free, covered by
dart test (10 cases). First slice of the drawing-card core (D-103); not
yet wired.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/src/draw/draw_doc.dart

@@ -0,0 +1,56 @@
+/// The drawing-card document model + envelope parser (T-318 / D-91 / D-103).
+///
+/// A `clide draw` payload is a JSON document. In PRIMITIVE mode it carries raw
+/// SVG (`svg` inline or `svgPath`); in TEMPLATE mode it names a `template`
+/// (`d2`, `icon`, `compare`, `image`, …) whose fields a registered handler
+/// lowers to SVG. Either way an optional card-level `label`/`description`
+/// renders as a caption beneath the drawing (D-103: SVG is the substrate; a thin
+/// Flutter overlay carries the chrome).
+///
+/// This is just the typed envelope + a tolerant parser — template lowering and
+/// painting live elsewhere. Never throws; a non-object payload yields `null`.
+///
+/// Flutter-free: pure Dart, runs under `dart test`.
+library;
+
+/// A parsed drawing-card document.
+class DrawingCardDoc {
+  const DrawingCardDoc({this.label, this.description, this.template, this.svg, this.svgPath, this.fields = const {}});
+
+  /// Card-level caption, shown beneath the drawing when present.
+  final String? label, description;
+
+  /// Template name (`d2`, `icon`, …). `null` or `svg` ⇒ primitive mode.
+  final String? template;
+
+  /// Primitive-mode SVG: inline source, or a path to an `.svg`.
+  final String? svg, svgPath;
+
+  /// The full document map, so a template handler can read its own fields
+  /// (`source`, `items`, `path`, …).
+  final Map<String, Object?> fields;
+
+  /// True when the card is raw SVG rather than a named template.
+  bool get isPrimitive => template == null || template == 'svg';
+}
+
+/// Parse a decoded-JSON drawing-card document (a `Map`). Returns `null` only
+/// when [json] isn't a JSON object; never throws.
+DrawingCardDoc? parseDrawingCardDoc(Object? json) {
+  if (json is! Map) return null;
+  final card = json['card'];
+  final cardMap = card is Map ? card : const {};
+  return DrawingCardDoc(
+    label: _str(cardMap['label']) ?? _str(json['label']),
+    description: _str(cardMap['description']) ?? _str(json['description']),
+    template: _str(json['template']),
+    svg: _str(json['svg']),
+    svgPath: _str(json['svgPath']),
+    fields: {
+      for (final e in json.entries)
+        if (e.key is String) e.key as String: e.value,
+    },
+  );
+}
+
+String? _str(Object? v) => v is String && v.isNotEmpty ? v : null;