feat(sync): add a reliable inbound sync controller

[marcus-pousette]

What

Add createInboundSync, a small app-facing controller for reconciling and subscribing the same filters across registered peer transports.

• Register, replace, remove, or clear peers by stable id.
• Run one-shot reconciliation across selected peers and filters.
• Keep declarative live subscriptions aligned with the current peer/filter set.
• Expose busy, peer, desired-filter, and active-subscription status.
• Report target context to optional status/error observers.

This is the controller-only layer of the former #138. Protocol cancellation lives in #204; playground adoption is a separate follow-up.

Failure contract

syncOnce resolves only when every requested peer/filter target succeeds. It lets all requested targets finish, then throws InboundSyncAggregateError when any failed. Each entry retains the exact peerId, filter, and original error, including unknown requested peers.

A timeout aborts the underlying protocol session rather than merely stopping the caller's wait.

Observer callbacks are notifications: exceptions from onError or onStatus cannot replace the aggregate result or corrupt controller state. Repeating the same declarative filter set restarts subscriptions that previously failed.

Fast paths left

• With no peers or filters changing, subscribe reuses the existing filter objects and only starts missing peer subscriptions.
• Duplicate filters and requested peer ids are deduplicated.
• Existing subscriptions are retained when a transport is unchanged.
• One-shot sync delegates directly to SyncPeer.syncOnce; the controller adds only target iteration, status, timeout, and aggregation.
• No transport discovery or background scheduler is introduced.

Verification

• Combined @treecrdt/sync stack: 31/31 tests.
• Focused inbound-controller suite: 12/12.
• Regressions cover partial failure, missing peers, real cancellation, retrying identical failed subscriptions, transport replacement, synchronous startup failure, observer exceptions, close/cleanup, and busy-state restoration.

Dependency

Stacked on #204. Review only this five-file controller diff.