add anyhow for errors, main does a demo

[Idleness76]

As a demo, main now does this:

1. Build a multi-step graph: Start -> A -> B -> End (using NodeKind::Other("A"), NodeKind::Other("B")) plus a duplicate edge from Start to A to hit de-dup logic in frontier building.
2. Run App::invoke on a richly populated initial state (seed meta, extra, maybe an initial output) to show version increments across channels.
3. After invoke, manually exercise reducers on cloned data to show direct usage (ADD_MESSAGES, APPEND_VEC, MAP_MERGE).
4. Manually call apply_barrier with crafted NodePartial sets:
5. One with mixed channel updates.
6. One with empty vectors/maps (no version bump).
7. One where you preset messages.version = u64::MAX then add a message to demonstrate saturating_add behavior (stays at MAX).
8. Demonstrate both GraphBuilder error cases (MissingEntry and EntryNotRegistered) and print them.
9. Show snapshot() before and after a manual mutation to confirm independence.
10. Return Result<()> from main, propagate errors instead of unwrap where possible.
11. Add small helper fn to pretty-print VersionedState to keep main readable.