Dtype plot behaviour

[Bilchreis]

Summary

closes #85

• Introduced a DTypePlot behaviour with four implementations (Scalar, Enum, ArrayHeatmap, Struct) that encapsulate how each data type builds ECharts series, options, and live-update payloads, replacing the scattered dtype-branching in Builders
• Added multi-grid struct plots: one stacked grid row per plottable field, with per-field dtype dispatch, shared x-axis pointer linking, and a scalar legend positioned at the vertical centre of scalar sub-fields
• Improved live plot data management in the JS hook: time-window-based trimming replaces max-point-count trimming, and expanding the view window triggers a fetch-chart-range push to backfill historical data via a new prepend-chart-data-* event; shrinking eagerly drops out-of-range points
• Added _xAxisPatch helper so range-button updates apply to all x-axes in multi-grid charts (struct subplots)
• Extended PlotSpec with a trace_updates/3 callback for typed live-update dispatch