feat(icon): add icon token registry + pipeline + export (UI Kit 5.0)

[Gr1dlock]

Stacked on #29 → #27. Review/merge those first; base retargets to main after.

What

Adds the full icon token system from the UI Kit 5.0 Iconography page: 187 icons × weights (dynamic/regular/solid, plus a few one-off weight4/disabled/growth) = 561 tokens. Wire token format: icon.{name}.{weight} (e.g. icon.arrow-right.regular).

How it works

• tokens/definitions/icon/icons.json — the registry (name → weight → Figma node id), source of truth + export manifest. Names normalized to kebab-case (shield 3→shield-3, _dollar→dollar, _coins/coins merged).
• tokens/formats/icons.ts + tokens/build/icons.ts — emit build/web/nucleus-icons.json (token → { path, size }) and copy present SVG sources to build/web/icons/{name}/{weight}.svg. Missing SVGs are reported, not fatal.
• tokens/scripts/fetch-icons.ts (npm run fetch:icons) — batch-exports every icon as SVG from Figma via the Images API into tokens/definitions/icon/svg/. Requires FIGMA_TOKEN.
• index.d.ts gains an IconToken literal union; web package files now publishes icons/**.

On the SVG assets

arrow-right (all 3 weights) is committed as a worked example so the pipeline is verifiable end-to-end. The remaining 558 SVGs are populated by running npm run fetch:icons with a Figma personal access token — there's no bulk-SVG export available through the design tooling otherwise, so the registry + script are the mechanism. Run it in CI or locally and commit the result (or I can run it if given a token).

npm run build, typecheck, lint, format:check all pass (build reports 558 missing until the export runs).

Follow-ups

• Native (iOS asset catalog / Android vector drawables) generation — the registry + node ids are platform-agnostic, so a per-platform generator can follow.
• A couple of names need a design sanity check (badge-not-checked, cellular-no-signal, the _-prefixed aliases).

Part of adopting Nucleus tokens in app-backend SDUI V4 (CTA style.icon).

🤖 Generated with Claude Code

[Gr1dlock]

Superseded by #25 (NucleusIcons by @Priva28), which ships the real SVG assets + iOS/Android/web outputs for all 141 icons × outline/regular/solid. Closing this thinner registry/pipeline PR in favor of #25.

Note for SDUI V4: #25 identifies icons by name + a separate variant and its published web index.d.ts stays Record<string,string>. To give consumers a typed token + a single wire string, we'd want an IconToken literal-union (icon.{name}.{variant}) generated from #25's icon set — happy to fold that into the TS-types PR (#27) or leave it to you.