[FCE-2925] Document @fishjam-cloud/composition usage

[Gawor270]

What

Expands the @fishjam-cloud/composition package README (previously a one-line stub) into a full usage guide.

Where

packages/composition/README.md in js-server-sdk.

Chose the package README because that is where the code lives and it matches the repo convention — every package here has a README, and the sibling composition-cli README already carries real usage docs. The documentation/website Docusaurus sites have no composition/smelter pages yet, so nothing to extend there.

Contents

• Install (via composition-cli init, or manual).
• Minimal template: a default-exported React component using usePeers() + Smelter Tiles/InputStream.
• Reacting to peers/events: usePeers/usePeer/useRoom/useSpeakingState, the PeerWithStreams role split (cameraStream/screenShareStream/customStreams), the Stream/TrackState shape, active-speaker example, and the eventBus for custom events.
• Host-facing ./core (compositionStore / CompositionStoreFeed) noted as not-for-template-authors.
• Build/bundle with composition-cli (single ESM dist/App.js, allowed-imports contract, default-export + size validation).
• How it connects to Fishjam/foundry (server-side Smelter worker composing forwarded room tracks into one output stream).

Everything is grounded in the package source (hooks.ts, types.ts, store.ts, eventBus.ts, index.ts), the CLI (init.ts, build.ts, validate.ts, smoke.ts, manifests/v1.ts), and the @swmansion/smelter exports — no invented APIs.

Open questions

• composition-cli not on main yet. This branch is based on origin/main, where the composition-cli package isn't merged. The guide references it by npm name (no broken repo-relative links) since it ships alongside; if it should stay on a feature branch, the build section may need adjusting.
• Docs-site coverage. Should this also land as a how-to page in the documentation Docusaurus site (e.g. docs/how-to/backend/)? Left out for now since there's no existing composition/smelter section to match.
• Exact wording of the foundry/host runtime boundary (who registers the composition output, transport of the final stream) — kept deliberately high-level; happy to tighten with the foundry team's preferred terminology.

[linear]

FCE-2925

[Gawor270]

Closing per project decision: the full @fishjam-cloud/composition usage guide (FCE-2925) will live in the documentation site (documentation.fishjam.io) as part of the new Smelter Cloud section, not in the package README. Keeping the package README minimal.