[ui] add the Chart layer: typed authoring spec lowering to SVG

[fwbrasil]

Problem

kyo-ui can render typed SVG, but there is no way to author a data visualization from a dataset. Today that means hand-computing scales, axes, layout, and legends and emitting raw Svg.* shapes, with no type-driven mapping of data fields to visual marks and no reactive or animated updates.

Solution

Add Chart, a pure immutable authoring spec (not a UI node) that lowers to an Svg.Root. The dataset fixes the row type, so field accessors like x = _.month infer with no annotations, and a Plottable typeclass derives the right scale per field (band scales for enums, linear for numbers, time for instants). Marks (bar, line, area, point, text, error bar, rule) are the visual vocabulary; axes, legends, layout, stacking, theming, scales, interaction, and transitions are applied by refinement methods that return a refined Chart. Because Svg.Root is itself HtmlContent, a lowered chart drops into any HTML container, and a chart bound to reactive data animates between states via SMIL.

The public surface is Chart.scala; the scale inference, axis, layout, legend, mark, interaction, and transition machinery lives in kyo.internal, so the reviewable API is small relative to the diff.

Notes

• SVG layer: Animate gains calcMode/keyTimes/keySplines so chart transitions can drive spline-interpolated keyframes.
• Renderer correctness: the session id and base path are interpolated into a <script> JS string but were escaped with the HTML-attribute escaper, the wrong context. They now use a JS-string escaper. Both values are server-controlled today (a random UUID and a configured base path), so this is hardening, not a vulnerability fix; the escaping is simply now correct for the context.
• Demos renamed to the *Demo.scala convention, matching kyo-browser.
• Test cleanup: removed compile-assertions that only restate language guarantees (a sealed type can't be extended, a private member isn't callable); every genuine designed-rejection contract test is kept.
• Svg.Paint's Style.Color conversion moves from a given Conversion to a plain implicit def, dropping the now-unneeded import scala.language.implicitConversions across the module.