Add a support for Highcharts rendering backend #141

Motivation

Flint compiles one semantic chart input to multiple render targets (Vega-Lite, ECharts, Chart.js, Plotly, Excel, Image-Charts). Highcharts is widely deployed and exposes a serializable option object, so it's a natural additional target for hosts already using Highcharts. Adding it lets the same Flint input drive a Highcharts render without a separate authoring model.

Proposal

Add a highcharts backend to flint-js:

• assembleHighcharts(input) compiles the shared core semantic layer into a Highcharts option, exported from the package root and from a new flint-chart/highcharts subpath.
• Follow docs/adding-a-backend.md and mirror the existing backend structure (assemble → templates → instantiate-spec).
• Reuse Phase 0 semantic resolution and banded-axis overflow filtering.
• Register with validateChart; wire the MCP server and site editor.
• Add an interactive renderer so interaction_spec presets mount.

Scope (chart types)

Scatter, Bar, Grouped Bar, Stacked Bar, Histogram, Line, Area, Pie, Doughnut, Sunburst, Radar, Heatmap, Waterfall (13). Unsupported types are out of scope by design; the reference documents exactly what is ported.

Acceptance criteria

• Bar, line, area, scatter render on the standard gallery matrices
• tests/smoke.test.ts passes for the new assembler
• npm run typecheck and npm run test pass at repo root
• A dedicated test-data generator exercises backend-specific options

Notes

• Highcharts is an optional peer dependency (^11 || ^12 || ^13); externalized by the build.
• Implementation PR: feat(highcharts): add Highcharts rendering backend #140