Add JSON Schema generation for Flint ChartAssemblyInput

[yelper]

Summary

Adds a generation script (scripts/gen-json-schema.ts) that produces JSON Schema files describing the full permitted ChartAssemblyInput shape. These schemas enable LLM structured-output validation, IDE autocomplete for JSON-based workflows, and runtime validation.

This PR is just for feedback for the implementation; we'd need to wire it up to the build scripts and publicize it somewhere on the docs site.

If we actually wanted to do versioning (i.e., the $schema determines how Flint parses the given spec), then we'd need to publish these schemas in a well-known place. For Microsoft, this would be in the under the "http://developer.microsoft.com/json-schemas" URI, which is governed by https://github.com/microsoft/json-schemas/.

Output

Five schema files in docs/schema/:

File	Scope	Chart types
flint-input.schema.json	Universal (all backends)	18
flint-input-vegalite.schema.json	Vega-Lite	35
flint-input-echarts.schema.json	ECharts	37
flint-input-chartjs.schema.json	Chart.js	21
flint-input-plotly.schema.json	Plotly	38

Schema features

• Full ChartAssemblyInput structure (data, semantic_types, chart_spec, options, field_display_names)
• chartType enum - precise per backend; universal schema only includes types common to all four backends
• Per-chart-type conditional sub-schemas (allOf + if/then) constraining valid encoding channel names and typed chartProperties
• AssembleOptions with all layout tuning parameters fully typed
• Semantic type enum (40+ registered types) + SemanticAnnotation object form
• Root $ + schema property allowed for standalone .json files
• JSON Schema Draft 2020-12

Usage

npm run gen:schema

Example usage (by users)

Enables per-chart autocomplete suggestions / validation.

Open questions

• Should the generated schemas be checked in (current approach) or only produced in CI?
• Any chart properties missing from the conditional sub-schemas?
• Should we require encodings per-chart type? For example, it doesn't make sense to choose "Bar chart" but then not chose both and x + y encoding.
• Should we wire gen:schema into a pre-commit or CI check to keep schemas in sync?