cuprated: add node event streaming API (Node::events())

[orrinfrazier]

Closes #20

Summary

• Adds a NodeEvent enum and Node::events() returning a subscribable NodeEventListener, so embedders of the cuprated library can observe node activity (new blocks, reorgs) without polling.
• Implements the event-streaming checklist item of the Tracking Issue for Cuprate as a library Cuprate/cuprate#616 tracking issue and the cuprated as a library Cuprate/cuprate#554 API proposal — the gap left after Node::launch() (cuprated: embeddable node library with Node::launch() Cuprate/cuprate#592), where embedders held service handles but had no push-based notifications.

Changes

Piece 1 — event types + Node plumbing (events.rs, lib.rs)

• New events module: NodeEvent { NewBlock { height, hash }, Reorg { split_height, new_top_hash, new_chain_height } }; a pub(crate) NodeEventSender wrapping tokio::sync::broadcast (capacity 256); a public NodeEventListener (recv / try_recv / resubscribe).
• The sender is created once in launch(), stored in LaunchContext, and Node::events() returns a fresh, forward-looking listener.

Piece 2 — manager publishes events (manager.rs, manager/handler.rs)

• BlockchainManager gains a node_events sender (clone of the launch sender — single shared channel).
• NewBlock is published from add_valid_block_to_main_chain only for live BlockSource::Incoming blocks (peer relay or RPC submit_block) — mirroring the existing peer-broadcast filter.
• Reorg is published from try_do_reorg on the success arm.

Design decisions

• broadcast, not watch — discrete events with multiple independent consumers; watch would drop blocks.
• NewBlock excludes batch-sync and reorg re-applies — initial-sync progress is already observable via the existing node.syncer handle (target_height() / wait_for_synced()), and a reorg surfaces as a single Reorg event. This keeps the stream low-noise and non-overlapping.
• Deferred by design (rationale): SyncProgress/Synced (already pollable via SyncerHandle) and PeerConnected/Disconnected (no clean source exists in cuprated — would require cuprate-p2p changes). Good follow-ups once there's demand.

Testing

• 7 new tests + the 3 pre-existing reorg tests pass (18 lib tests + 2 doc-tests green).
• Event-API unit tests: prior subscriber receives, multiple subscribers, no-subscriber is a no-op, late subscriber misses prior events (forward-looking), capacity constant.
• Manager integration tests (via the existing mock_manager harness): adding a live block emits exactly one NewBlock{ height: 1, hash }; a reorg emits a Reorg{ split_height: 2, .. } and no NewBlock for re-applied blocks.
• cargo clippy --package cuprated --all-targets -- -D warnings and cargo fmt --check clean. No new dependencies (tokio already present).

How to test

let node = Node::launch(config).await?;
let mut events = node.events();
while let Ok(ev) = events.recv().await {
    match ev {
        NodeEvent::NewBlock { height, hash } => { /* ... */ }
        NodeEvent::Reorg { split_height, new_chain_height, .. } => { /* ... */ }
    }
}

Reviewer notes

Reviewed via 3-way cross-family consensus (Claude + Gemini + Codex) with an opus adjudicator: 0 blockers. One low doc nit was accepted and applied (the NewBlock doc now notes Incoming includes locally-submitted RPC blocks, not only peer relay). One finding (a claim that resubscribe() inherits lag state) was rejected — tokio's resubscribe is forward-looking from the current tail.

Related: Cuprate#616 (tracking), Cuprate#554 (API proposal), Cuprate#592 (merged launch).