validate_chart silently accepts nonexistent chartProperties and options keys

Summary

validate_chart silently accepts nonexistent chartProperties and options keys — it returns valid: true, warnings: []. Agents (and humans) get no signal that a knob does not exist, so they retry blindly.

Verified against flint-chart@0.3.0. Source refs at main (commit 95b2552).

Repro

{
  "chart_spec": {
    "chartType": "Line Chart",
    "encodings": { "x": { "field": "x" }, "y": { "field": "y" } },
    "chartProperties": { "lineWidth": 5, "pointSize": 12, "totallyFakeKnob": true }
  },
  "options": { "fontSize": 18, "bogusOption": "xyz" }
}

→ { "valid": true, "warnings": [], "errors": [] }

None of lineWidth, pointSize, totallyFakeKnob, fontSize, bogusOption exist for a Line Chart, yet nothing is reported.

Root cause

packages/flint-js/src/core/normalize-properties.ts (normalizeChartProperties) validates discrete property values against the template's declared options, but never checks whether a chartProperties key exists in chartTemplate.properties. options keys are not validated at all. No unknown-key check exists in the pipeline.

Suggested fix

In normalizeChartProperties (and/or the validate path), diff the incoming keys against the template's known option keys (chartTemplate.properties.map(p => p.key) — the same model that drives the customization panel) and the recognized AssembleOptions names, and push a warning per unknown key:

unknown chartProperty "lineWidth" for Line Chart; available: [cornerRadius, ...]

Keep it a warning, not an error, for forward compatibility.

Start in packages/flint-js/src/core/normalize-properties.ts at normalizeChartProperties, then trace the validate path and the chart template properties model. Run the supplied validate_chart reproduction and inspect how AssembleOptions names are represented. Done means unknown chartProperties and options produce individual warnings while recognized keys and forward-compatible inputs remain valid.