feat(svg): XML tokenizer + inline-style normalizer for the drawing card (T-320)

Zero-dependency, tolerant XML reader (elements/attrs/text/comments/prolog/
CDATA/entities; <style> read as raw text) producing a generic element tree.
The normalizer folds d2/graphviz's class-based <style> rules into inline
presentation attributes — cascade presentation-attr < tag < class < style —
then drops <style>/class/style, so the painter only ever sees inline attrs
(D-103). Flutter-free, covered by dart test (26 cases). No user-visible
behaviour yet.

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

test/svg/svg_xml_test.dart

@@ -0,0 +1,98 @@
+import 'package:clide/src/svg/svg_xml.dart';
+import 'package:test/test.dart';
+
+void main() {
+  XmlElement parse(String s) {
+    final root = parseXml(s);
+    expect(root, isNotNull, reason: 'expected an element root for: $s');
+    return root!;
+  }
+
+  group('parseXml', () {
+    test('element with attributes', () {
+      final e = parse('<rect x="1" y="2" width="3" height="4"/>');
+      expect(e.name, 'rect');
+      expect(e.attrs, {'x': '1', 'y': '2', 'width': '3', 'height': '4'});
+      expect(e.children, isEmpty);
+    });
+
+    test('single-quoted and unquoted attribute values', () {
+      final e = parse("<g fill='red' opacity=0.5/>");
+      expect(e.attrs['fill'], 'red');
+      expect(e.attrs['opacity'], '0.5');
+    });
+
+    test('nested children preserve order', () {
+      final e = parse('<g><rect/><circle/></g>');
+      expect(e.name, 'g');
+      expect(e.children.whereType<XmlElement>().map((c) => c.name), ['rect', 'circle']);
+    });
+
+    test('text content is captured', () {
+      final e = parse('<text>hello</text>');
+      final t = e.children.single as XmlText;
+      expect(t.text, 'hello');
+    });
+
+    test('comments are skipped', () {
+      final e = parse('<g><!-- a comment --><rect/></g>');
+      expect(e.children.whereType<XmlElement>().map((c) => c.name), ['rect']);
+      expect(e.children.whereType<XmlText>(), isEmpty);
+    });
+
+    test('xml prolog and doctype are skipped', () {
+      final e = parse('<?xml version="1.0"?><!DOCTYPE svg><svg width="10"/>');
+      expect(e.name, 'svg');
+      expect(e.attrs['width'], '10');
+    });
+
+    test('CDATA content is captured verbatim', () {
+      final e = parse('<style><![CDATA[ .a { fill: red } ]]></style>');
+      expect((e.children.single as XmlText).text, contains('.a { fill: red }'));
+    });
+
+    test('entities are decoded in text and attributes', () {
+      final e = parse('<text title="a &amp; b">&lt;tag&gt; &#65;</text>');
+      expect(e.attrs['title'], 'a & b');
+      expect((e.children.single as XmlText).text, '<tag> A');
+    });
+
+    test('style body is read as raw text, not markup', () {
+      // CSS with > and { } must not be parsed as elements.
+      final e = parse('<style>.edge > .head { fill: #0D32B2 } .b{stroke:none}</style>');
+      expect(e.name, 'style');
+      final css = (e.children.single as XmlText).text;
+      expect(css, contains('.edge > .head'));
+      expect(css, contains('stroke:none'));
+    });
+
+    test('namespaced attribute names are kept verbatim', () {
+      final e = parse('<image xlink:href="a.png" href="b.png"/>');
+      expect(e.attrs['xlink:href'], 'a.png');
+      expect(e.attrs['href'], 'b.png');
+    });
+
+    test('mismatched close tag is tolerated, no throw', () {
+      final e = parse('<g><rect></wrong></g>');
+      expect(e.name, 'g');
+      expect(e.children.whereType<XmlElement>().single.name, 'rect');
+    });
+
+    test('descendants walks the whole subtree', () {
+      final e = parse('<svg><g><rect/></g><circle/></svg>');
+      final names = e.descendants().whereType<XmlElement>().map((c) => c.name).toList();
+      expect(names, ['g', 'rect', 'circle']);
+    });
+
+    test('non-element root returns null, never throws', () {
+      expect(parseXml(''), isNull);
+      expect(parseXml('   '), isNull);
+      expect(parseXml('just text'), isNull);
+    });
+
+    test('unterminated tag does not throw', () {
+      // Should not hang or throw; returns whatever was understood.
+      expect(() => parseXml('<svg><rect x="1"'), returnsNormally);
+    });
+  });
+}