feat(viewer): add controlled selection highlight

[LucaVor]

What changed

• adds a typed selection?: DataPointID[] | null prop to the high-level EmbeddingAtlas API
• initializes and reactively updates the existing shared highlight store
• defensively copies caller arrays, preserves caller order, deduplicates ordered-equal updates, and supports clearing with null/undefined
• adds focused store-level coverage for initialization, update, deduplication, copying, and clearing

Why

The dashboard already has an internal point-highlight store, but the high-level viewer API did not expose a controlled input. Embedders therefore could not mirror an external selection into the embedding and instances views without reaching through private internals.

Validation

• npm run test -w @embedding-atlas/viewer — 3 files / 36 tests passed
• focused selection test — 3/3 passed
• Prettier on all four changed paths — passed
• git diff --check — passed

The repository's canonical GitHub Actions workflow will run its full Rust/WASM build, workspace type checks, complete tests, formatting, and Clippy. This is opened as a draft pending that result.