SeriesChart: chart-series contract and single-variable chart plugin

[BhattaraiSijan]

Summary

Adds the shared chart-series contract (src/essence/Tools/_shared/types/chartSeries.ts) and SeriesChart, a generic chart panel rendering it. SeriesChart knows nothing about data sources: any plugin listed in its sources tool variable that emits plugin:<id>:seriesLoading/seriesReady/seriesError/seriesCleared gets a chart card per chartId — loading, error, and ready states included. Payloads are treated as untrusted: malformed ones warn and drop rather than crash the panel.

Rendering (ECharts, already a dependency): one variable visible at a time (air-quality-domain convention, e.g. OpenAQ), with the single-select legend as the parameter picker — the y-axis renames to the picked variable's unit. Wheel + slider zoom (slim theme-primary scrubber), reset button in the card header, crosshair tooltips. All time labels are UTC by design: epoch-value axis with our own formatters, since ECharts' native time axis renders viewer-local. All colors route through --theme-* tokens (canvas via a resolved theme object; spec-asserted so literals can't regress in).

The default sources: ["fetch-timeseries"] forward-references the fetcher plugin from the stacked PR — inert until it merges.

Size note: ~360 lines are unit tests and ~230 are styles/config/docs; reviewable logic is ~600 (chartData.ts, panel, adapter).

Merge order

1. Emit feature:click on the bus for every vector feature click #292 feature:click core event (independent of this one)
2. this PR → development
3. FetchTimeseries: feature clicks to chart-series events #294 FetchTimeseries (stacked on this PR)

Testing

Contract guards and option-builder covered by tests/unit/chartSeries.spec.js + tests/unit/seriesChartData.spec.js (UTC handling, single-select legend, unit-named axis, themed scrubber, tooltip formatting). Full unit suite green. Manual recipe lives in the stacked FetchTimeseries PR, which provides the first emitter.