Add inbound sync API

[marcus-pousette]

Summary

Adds the inbound companion to #137: createInboundSync({ localPeer }) owns one-shot reconciliation and live subscriptions by filter across current and future peer transports.

The playground now declares desired inbound scopes (all or children(parent)) and uses the same inbound helper for manual scoped sync and live subscriptions, instead of maintaining separate per-peer subscription maps and manual peer success/error loops.

Before

const sub = peer.subscribe(transport, { children: { parent } }, opts)
await sub.ready
sub.done.catch(handleLiveError)
sub.stop()

for (const peerId of targets) {
  await peer.syncOnce(transport, filter, opts)
}

After

const inbound = createInboundSync({ localPeer: peer })
inbound.addPeer(peerId, transport)

const children = inbound.scope({ children: { parent } })
await children.syncOnce()
children.startLive()
children.close()

Stacked on #137.