cuprated: event-driven block synchronization

[redsh4de]

What

Make blockchain synchronization event-driven instead of polling every 30s. This makes new sync initiation near-instant as it respond to relevant events. Closes #532.

Why

The syncer previously polled every 30 seconds to check if we are behind our peers. This meant up to a 30-second delay before reacting to network's cumulative difficulty changes. Also unnecessary work when nothing had changed

Where

• p2p/p2p-core/src/client.rs - new PeerSyncCallback struct
• p2p/p2p-core/src/client/handshaker.rs, builder.rs, request_handler.rs, timeout_monitor.rs - plumbing
• p2p/p2p/src/lib.rs, inbound_server.rs, connection_maintainer.rs - notify syncer on 0 peer -> 1+ peer transitions
• consensus/context/src/lib.rs - add a read function for our cumulative difficulty for use in the callback
• binaries/cuprated/src/main.rs, p2p.rs, p2p/request_handler.rs, blockchain/manager.rs, blockchain/syncer.rs - wire everything together, add loop label for clarity for breaks

How

Replace the 30-second polling interval with a PeerSyncCallback struct that wraps a tokio::sync::Notify with three waking paths:

• Filtered wake (call): When a peer reports new sync data, wake the syncer only if the peer's cumulative difficulty exceeds ours.
• Unconditional wake (wake_unconditionally): When we receive an orphan block, wake the syncer immediately.
• First-peers wake (wake_on_first_peers): When transitioning from 0 to 1+ connected peers, wake the syncer immediately. Uses a guard that the syncer re-arms (wake_on_first_peers_arm) each time it enters the NoPeers state, so the mechanism works across repeated complete disconnect -> connect cycles.

The syncer checks sync status, then either awaits a notification (Synced/NoPeers) or starts the inner block downloader loop (BehindPeers).

After the downloader loop ends, the outer syncer loop re-checks status again before parking. The synced_notify for the Tor P2P zone start is now fired only once on initial sync completion rather than repeatedly.