feat: chart interactivity, non-finite robustness, and full API docs (0.3.0)

[AndroidPoet]

Summary

Makes the Flutter charting library interactive and hardens it to ship-grade. Three pillars: interactivity (opt-in, no new deps), non-finite robustness (real crash fixes), and complete public-API docs + a much larger test suite. Bumps to 0.3.0.

Interactivity

Render-only charts can now be made interactive by wrapping any cartesian renderer:

InteractiveChart(
  renderer: LineChartRenderer(points: data),
  interaction: ChartInteraction(
    tooltip: true,
    selection: true,
    rangeSelection: true,
    onSelected: (sel) => ...,
    onRangeSelected: (range) => ...,
  ),
)

• New InteractiveChart wrapper + ChartInteraction config — trackball/tooltip, tap-to-select, drag range-select, all painted on the same Canvas (reuses the reveal animation, zero new dependencies).
• New retained-geometry layer: InteractiveRenderer interface, ChartScene/PlotMark, reversible CartesianScale, ChartHitTest, and a LabelLayout de-overlap helper. Paint geometry and hit-test geometry share one scale so they can't drift.
• Cartesian renderers implement buildScene(). ChartRenderer/InteractiveRenderer are re-exported, and a duration knob was added to every chart widget (defaults preserve prior behavior).
• Non-interactive / non-cartesian renderers degrade gracefully (no-op).

Robustness

• Added drafterFinite / drafterFiniteOrNull sanitizers applied at value ingestion across all renderers. This fixes release-mode crashes where NaN/Infinity reached .toInt() (histogram bins, bubble nice-number rounding, axis steps), plus 5 bar-chart buildScene non-finite centers caught by the new tests.
• InteractiveChart.didUpdateWidget resets the cached scene + interaction state on renderer swap (no stale highlight); _publish is guarded against post-dispose pointer events; hit-test + CartesianScale.nearestIndex guard non-finite pixels.

Docs & quality

• Documented every public member and enabled public_member_api_docs as an enforced gate — a clean analyze now proves doc completeness.
• @immutable on all data classes (intentionally not on renderers that hold size-keyed memo caches).
• Tests 57 → 196 (robustness_test.dart matrix: empty/single/all-equal/negative/NaN/Inf across every renderer + all widgets; interaction_behavior_test.dart: hover/tap/drag/dispose/renderer-swap/scene-cache). Line coverage 96.3%.

Verification

• flutter analyze — No issues found (with public_member_api_docs on)
• flutter test — 196/196 pass
• dart format — clean
• dart pub publish --dry-run — only the benign uncommitted-files notice