research: visualisation library for the SmartEM front end - comparative analysis and ADR

Updated. The analysis this issue calls for is still wanted, and the candidate list and
comparison axes below still stand. Three things have changed since it was written, and the
scope has widened as a result.

## What changed

**The blocking relationship no longer exists.** This issue stated that implementation blocks on
the ADR landing. It did not: smartem-frontend#67 closed on 2026-06-15 with the model-weight
display shipped.

**No library was chosen, and none was added.** `apps/smartem/package.json` contains no charting
dependency of any kind. The weight display, and everything since, was hand-rolled in SVG.

**The ADR number is taken.** `0018` is now `0018-smartem-agent-keycloak-auth.md`, and the
sequence has reached `0021-micrograph-preview-image-ingestion-and-serving.md`. The next
available number is **0022**.

## The situation this issue warned about has arrived by a different route

The original rationale was that choosing a library casually creates a de-facto standard and
therefore lock-in. What happened instead is that *not choosing* became the de-facto standard.
Hand-written `