feat(echarts): chart_spec.echarts, a native option escape hatch

[windylcx]

What

chart_spec.echarts — a native ECharts option patch that is merged onto the compiled option, plus column binding for series added through it.

This closes the gap where a chart needs something the semantic layer does not model. A second value axis, a bar and a line in one chart, markLine / markArea, dataZoom, axis or tooltip formatters, per-series areaStyle, custom grids — today a caller who needs one of those has to leave Flint and hand-write the option, losing the layer they came for.

{
  "chartType": "Line Chart",
  "encodings": { "x": { "field": "month" }, "y": { "field": "revenue" } },
  "echarts": {
    "legend": { "show": true, "top": 6 },
    "yAxis": [{ "name": "Revenue" }, { "name": "Margin", "position": "right" }],
    "series": [
      { "name": "Revenue", "lineStyle": { "width": 2 } },
      { "type": "line", "field": "margin", "name": "Margin rate", "axis": "right" }
    ]
  }
}

Applied last, so it is authoritative over what Flint decided above it: objects merge recursively, arrays of plain objects (series, xAxis, yAxis) merge element-wise by index with entries past the end appended, any other array replaces, and _-prefixed keys are dropped with an info warning. A series entry that binds a column is added (and axis: "right" puts it on a right-hand value axis that Flint creates if missing); one that does not patches the series at its own index. Backend-scoped: the other assemblers ignore the key, as Vega-Lite ignores theme_spec. No template change and no new channel.

Guide: docs/native-echarts-options.md (+ zh-CN); docs/api-reference.md and the CHANGELOG updated. The MCP tool input schema declares the field too, since zod drops unknown keys in chart_spec.

Verified: 18 new flint-chart tests pinning the merge contract, 3 flint-mcp tests rendering a column-bound dual-axis chart end to end, plus a compile_chart passthrough case; typecheck, lint, test, build and site:build all clean.

[windylcx]

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