Playground extends

[dlrice]

Purpose

Surface the extend-uniprot Starter Kit recipe as a playground preset so the extends: feature demos on the hosted site. The existing extend-default example extends /src/default-config.yaml, which only resolves under the dev server. Also fixes supporting docs/blog copy.

Approach

• New extend-uniprot preset loads the recipe verbatim (?raw); its extends: is the version-pinned jsDelivr dist/default-config.yaml, fetched over the network at render time, so it resolves on the hosted page. Its ./data/ sample is repointed at the served hotspots.csv.
• Deep-linked the tutorial "Try it live" callout to #preset=extend-uniprot.
• Corrected the blog's Starter Kit paragraph: the config check is a schema shape check — dropped the inaccurate "prevents an empty track" claim.

Testing

src/playground/__spec__/presets.spec.ts loads every preset (now including extend-uniprot) through loadConfig, mocking the jsDelivr extends: fetch with this repo's src/default-config.yaml (mirrors starter-kit.spec.ts; throws on any non-http ref so a dev-only /src/ path can't sneak back in). Broadened the served-data assertion to not.toMatch(/data:\s*\.\//) so no bare relative data: path can survive substitution.

Run: npx vitest run src/playground/__spec__/presets.spec.ts

Visual changes

New "Extend UniProt (your track + the full default viewer)" entry in the playground preset picker; selecting it renders a custom track layered on the full default viewer.

Checklist

• My PR is scoped properly, and "does one thing only"
• I have reviewed my own code
• I have checked that linting checks pass and type safety is respected
• I have checked that tests pass and coverage has at least improved, and if not explained the reasons why
• If needed, the changes have been previewed by all interested parties.