feat(svg): typed scene model + document builder for the drawing card (T-320)

The builder ties the five parsers together: parseXml → inlineStyles →
a typed SvgNode tree (group/rect/ellipse/line/poly/path/text/image) with
viewBox, per-node Affine transforms, and inheritance-flattened SvgStyle
(fill/stroke/font resolved to ARGB; opacity per-node). Tolerant — a
non-svg root yields an empty doc; defs/marker deferred. An end-to-end test
runs the whole pipeline against a real d2-rendered SVG fixture. Flutter-
free, covered by dart test (17 cases). No user-visible behaviour yet.

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

lib/src/svg/svg_node.dart

@@ -0,0 +1,131 @@
+/// The typed SVG scene model the painter draws (T-320 / D-103).
+///
+/// The document builder ([buildSvgDocument]) lowers a normalized XML tree into
+/// this model: every node carries a resolved [SvgStyle] (inheritance already
+/// flattened), an optional [Affine] transform, and typed geometry. Colours are
+/// packed ARGB ints, not `dart:ui` Colors, so the model stays Flutter-free and
+/// `dart test`-able; the painter converts.
+///
+/// `null` style fields mean "unspecified" — the painter applies the SVG default
+/// (fill black, stroke none, stroke-width 1).
+library;
+
+import 'svg_path.dart';
+import 'svg_transform.dart';
+
+enum SvgLineCap { butt, round, square }
+
+enum SvgLineJoin { miter, round, bevel }
+
+enum SvgTextAnchor { start, middle, end }
+
+enum SvgBaseline { auto, middle, hanging }
+
+/// Resolved presentation style for a node, with inheritance already applied.
+class SvgStyle {
+  const SvgStyle({
+    this.fill,
+    this.stroke,
+    this.strokeWidth,
+    this.opacity = 1.0,
+    this.fillOpacity,
+    this.strokeOpacity,
+    this.dashArray,
+    this.lineCap,
+    this.lineJoin,
+    this.fontFamily,
+    this.fontSize,
+    this.fontWeight,
+    this.textAnchor,
+    this.baseline,
+  });
+
+  /// The root inheritance context: nothing specified, full opacity.
+  static const initial = SvgStyle();
+
+  final int? fill; // ARGB; 0x00000000 = explicit none
+  final int? stroke; // ARGB
+  final double? strokeWidth;
+  final double opacity; // element/group opacity — NOT inherited
+  final double? fillOpacity;
+  final double? strokeOpacity;
+  final List<double>? dashArray;
+  final SvgLineCap? lineCap;
+  final SvgLineJoin? lineJoin;
+  final String? fontFamily;
+  final double? fontSize;
+  final int? fontWeight; // 400, 700, …
+  final SvgTextAnchor? textAnchor;
+  final SvgBaseline? baseline;
+}
+
+/// A node in the typed scene — group or leaf. [style] is fully resolved;
+/// [transform] is `null` when identity.
+sealed class SvgNode {
+  const SvgNode(this.style, this.transform);
+  final SvgStyle style;
+  final Affine? transform;
+}
+
+class SvgGroup extends SvgNode {
+  const SvgGroup(super.style, super.transform, this.children);
+  final List<SvgNode> children;
+}
+
+class SvgRect extends SvgNode {
+  const SvgRect(super.style, super.transform, this.x, this.y, this.width, this.height, this.rx, this.ry);
+  final double x, y, width, height, rx, ry;
+}
+
+/// Circle is an ellipse with `rx == ry`.
+class SvgEllipse extends SvgNode {
+  const SvgEllipse(super.style, super.transform, this.cx, this.cy, this.rx, this.ry);
+  final double cx, cy, rx, ry;
+}
+
+class SvgLine extends SvgNode {
+  const SvgLine(super.style, super.transform, this.x1, this.y1, this.x2, this.y2);
+  final double x1, y1, x2, y2;
+}
+
+/// Polyline (open) or polygon (`closed == true`). [points] is `[x0,y0,x1,y1,…]`.
+class SvgPolyline extends SvgNode {
+  const SvgPolyline(super.style, super.transform, this.points, this.closed);
+  final List<double> points;
+  final bool closed;
+}
+
+class SvgPath extends SvgNode {
+  const SvgPath(super.style, super.transform, this.segments);
+  final List<SvgPathSeg> segments;
+}
+
+class SvgText extends SvgNode {
+  const SvgText(super.style, super.transform, this.x, this.y, this.text);
+  final double x, y;
+  final String text;
+}
+
+class SvgImage extends SvgNode {
+  const SvgImage(super.style, super.transform, this.x, this.y, this.width, this.height, this.href);
+  final double x, y, width, height;
+  final String href;
+}
+
+/// `viewBox="minX minY width height"`.
+class SvgViewBox {
+  const SvgViewBox(this.minX, this.minY, this.width, this.height);
+  final double minX, minY, width, height;
+}
+
+/// A parsed SVG document: optional intrinsic [width]/[height] (px), optional
+/// [viewBox], and the [root] group.
+class SvgDocument {
+  const SvgDocument({this.width, this.height, this.viewBox, required this.root});
+
+  static const empty = SvgDocument(root: SvgGroup(SvgStyle.initial, null, []));
+
+  final double? width, height;
+  final SvgViewBox? viewBox;
+  final SvgGroup root;
+}