Declare `<pattern>` resources on a chart (hatch, dot and animated pattern fills)

Version: @tanstack/charts 0.16.0.

ChartSpec.gradients lets a definition declare linear gradients that the renderer emits into <defs> and marks reference as url(#id) paint, and the export path serialises them. There is no equivalent for <pattern>: a definition cannot declare a hatch, dot or stripe fill, and F-259 in API-FRICTION.md ("Chart resources cannot declare patterns") notes that injecting an application-owned pattern into the chart svg is not an intended workaround.

Without it, pattern fills (a common request for accessibility-friendly and print-friendly series encoding in Recharts, ECharts, Highcharts and Vega) have to be rendered by the host outside the chart svg and referenced by id, which breaks renderChartImage/serializeChartSvg and ties the fill to the DOM host. Concrete uses we are migrating: a funnel stage renderPattern(id, color) prop, sankey link patterns, hatched area fills on candlestick and gauge, heatmap cell hatch, and a loading placeholder whose sweep is an animated <pattern>.

Would it be possible to add a patterns resource next to gradients (id, width/height, patternUnits, and either a small set of presets such as lines/circles with stroke/fill/strokeWidth/angle, or a list of primitive shapes), emitted by the SVG renderer and the export path alike, so marks can use fill: 'url(#id)' natively? An optional svgAnimation-aware offset on the pattern would also cover animated sweeps on loading placeholders.

No implementation files or tests are named. Start by reading F-259 in API-FRICTION.md and tracing the existing gradients handling through the SVG renderer and export path. Done should include a defined patterns resource API that emits pattern definitions and preserves them through chart image rendering and SVG serialization.