Add LineChartNode, a data-driven line chart node

LineChartNode (type lineChart) and LineChartProperties: a series read from
variables['data'] through xField/yField (numbers or wall-clock dates), an
optional domain, plot insets, bands, y ticks, area gradient, area bands,
a polyline or Catmull-Rom curve, reference lines, x ticks, markers,
vertical lines and a scrub readout. List bindings and their JSON shapes
are documented on the class. Properties decode with defaults when
missing, and default text style paints have fixed ids so defaults compare
equal.

Register it with NodeJsonConverter, cover it in the every-node-type
round-trip test and add JSON round-trip and default tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01F3B1XCZZ7QGvZLUGgSG5H4

Author: claude

CHANGELOG.md

@@ -1,3 +1,11 @@
+## Unreleased
+
+- `LineChartNode` (`lineChart`) and `LineChartProperties`: a data-driven
+  line chart. The series, domain, bands, grid lines, area bands, reference
+  lines, x ticks, markers and vertical lines are bound through `variables`
+  (see the class docs for their JSON shapes); colors, insets, curve style,
+  text styles and the scrub readout are properties.
+
 ## 6.0.0
 
 ### Breaking changes

lib/src/api/node_json_converter.dart

@@ -66,6 +66,7 @@ class NodeJsonConverter implements JsonConverter<BaseNode?, Map> {
       'webView': WebViewNode.fromJson,
       'dropdown': DropdownNode.fromJson,
       'progressBar': ProgressBarNode.fromJson,
+      'lineChart': LineChartNode.fromJson,
       'variance': VarianceNode.fromJson,
       'listView': ListViewNode.fromJson,
       'gridView': GridViewNode.fromJson,