fix: warn about unknown chart input keys

[EuanTop]

Summary

• warn for unknown chartProperties and options keys across the Vega-Lite, ECharts, Chart.js, and Plotly assemblers
• derive accepted chart property keys from both the authored and effective templates, including property controls, encoding actions, pivot keys, and explicit template-only keys
• preserve forward compatibility by leaving unknown inputs untouched and keeping validation successful
• add regression coverage for all four assemblers and the MCP validate_chart tool

Fixes #68

Testing

• flint-js: 39 test files, 658 tests passed
• flint-mcp: 4 test files, 51 tests passed
• eslint: 0 errors (30 existing warnings)
• ESM, CJS, and declaration builds passed
• tsc --noEmit still reports the existing src/vegalite/templates/scatter.ts:338 implicit-any error from dev; this PR does not modify that file

[EuanTop]

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