DataViz: Rendering symbols for datapoints and legend items in panels

[WIP] **Problem 1**: we cannot currently do this in Grafana's core panels: Accessibility & visual diversity: <img width="484" alt="Image" src="https://github.com/user-attachments/assets/460d1795-37f3-414e-ba66-7016403edd4e" /> <img width="484" alt="Image" src="https://github.com/user-attachments/assets/c7788b92-2a3f-41d3-ae18-0200bb59c8cd" /> Visual clarity without referring to a large legend + mentally mapping shapes or colors to tangible items: <img width="484" alt="Image" src="https://github.com/user-attachments/assets/e60ff7dc-61cc-4a11-9e80-85de2a7010d7" /> --- in [[POC] TimeSeries: Rewrite exemplars renderer](https://github.com/grafana/grafana/pull/61594), i experimented with rewriting our existing Exmeplar markers from svg to Canvas, which addresses major performance issues with rendering large datasets as scatter points: <img width="1871" height="1094" alt="Image" src="https://github.com/user-attachments/assets/426fecc9-c9de-42e2-a89d-7a1f52e889b5" /> --- in [[WIP] StateTimeline: Support merging thresholds and value mappings](https://github.com/grafana/grafana/pull/99402) i'm adjusting the value mappings and threshold compiler to be re-usable across multiple visualizations, which will likely include icon mappings, like the Canvas panel supports: <img width="484" alt="Image" src="https://github.com/user-attachments/assets/8ca29c85-ab57-43f0-9724-633b9c40abe8" /> --- **Related issues** Grafana: - https://github.com/grafana/grafana/issues/34882 - https://github.com/grafana/grafana/pull/30192 - https://github.com/grafana/grafana/issues/93090 - https://github.com/grafana/grafana/issues/78088 (kinda) uPlot: - https://github.com/leeoniya/uPlot/issues/431 - https://github.com/leeoniya/uPlot/issues/601 - https://github.com/leeoniya/uPlot/issues/941 - https://github.com/leeoniya/uPlot/pull/1082 --- **Some resources** - https://d3js.org/d3-shape/symbol - https://observablehq.com/@jarrettmeyer/d3-symbols - https://observablehq.com/@d3/fitted-symbols?collection=@d3/d3-sh…