feat(call): SFU relay support via optional backend

[HexaField]

What

Adds SFU (Selective Forwarding Unit) support to the call module as an opt-in backend. When a host provides an SfuBackend through the store's deps, the call connects through a relay server instead of building N−1 direct peer connections.

The peer-to-peer mesh remains the default and the fallback — the SFU path activates only when a backend appears in deps, and falls back to mesh automatically if the SFU join fails.

Why

The mesh topology scales to about 4–6 participants before upstream bandwidth saturates. An SFU receives each participant's media once and forwards it selectively, shifting the bottleneck from the weakest uplink to the relay's bandwidth. This unblocks larger calls without changing anything about smaller ones.

How

New: sfu.ts

• SfuBackend — structural interface satisfied by SfuManager from @coasys/ad4m without importing it. The host constructs the manager (with neighbourhood proxy, room id, agent DID, ICE config) and passes it in. The call module carries no dependency on @coasys/ad4m.
• createCallSfu() — adapter that wraps an SfuBackend behind the existing CallMesh interface. Joins the SFU, maps participant events to stream/state callbacks, and handles track replacement on the single RTCPeerConnection.

Modified: store.ts

• sfuBackend?: SfuBackend added to CallStoreDeps
• topology signal ('mesh' | 'sfu') — exposes the active topology for the template
• qualityPreference signal + setQualityPreference() action — simulcast layer selection ('high' / 'medium' / 'low'), only meaningful in SFU mode
• join() branches: SFU path acquires media first (the SFU needs tracks for the SDP offer), then joins the relay; mesh path unchanged
• Automatic mesh fallback on SFU join failure
• Proper teardown of SFU state

Modified: index.ts

• Exports: createCallSfu, SfuBackend, SfuQuality, CallTopology
• Updated module description

Integration

A host wiring up SFU support:

import { SfuManager, resolveTopology } from '@coasys/ad4m';

// Resolve topology from the neighbourhood's SFU config
const { topology, sfuPeer, config } = await resolveTopology(
  neighbourhood, neighbourhoodUrl, participantCount
);

// If SFU, construct the manager and pass it as the backend
const sfuBackend = topology !== 'mesh'
  ? new SfuManager(neighbourhood, roomName, agentDid, neighbourhoodUrl, undefined, config)
  : undefined;

const store = createCallStore({ ...deps, sfuBackend });

What the user sees

• Topology indicator: store.topology() returns 'mesh' or 'sfu' — the host decides how to render this
• Quality preference: store.setQualityPreference('medium') — the host adds a toggle if desired
• Everything else (tiles, focus, media, screen share) works identically

Testing

• All 70 existing tests pass (mesh path unchanged)
• Type-check clean
• Lint clean (lint-staged ran automatically)

🤖 Generated with Claude Code

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