feat: integrate journal chain into node

[Fraser999]

Wires signet-journal-chain into SignetNode: every committed block is encoded as a Journal::V1, pushed into a store-less chain task, and streamed out over the new /journal WebSocket.

Blocked on https://github.com/init4tech/journal-service/pull/22. Stays in draft until that lands; the [patch.crates-io] entry will be removed from the workspace manifest at that point.

Summary

• New JournalConfig in signet-node-config (env-var configurable; defaults 64 MiB / 200 entries / 100-journal subscriber lag).
• SignetNode owns the chain handle, a bounded mpsc sender, and an in-memory rolling previous_journal_hash. Main loop uses tokio::select! to observe the chain ingestion task alongside host notifications.
• signet-rpc serving layer takes an Option<JournalChainHandle>. When set, HTTP and WS transports merge journal_router and shut down via a shared CancellationToken with a 5s watchdog.

Follow-up (ENG-2017)

previous_journal_hash is in-memory only - not persisted, not rewound on revert. First journal after any restart or host revert is rejected by the chain. Ten reorg tests are #[ignore]d until this is fixed.

[Fraser999]

• feat: integrate journal chain into node #146 👈 (View in Graphite)
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