fix(consensus): open the store only after the execution engine is reachable

[ZanCorDX]

Fixes #466

App::start() opens the redb store before connecting to the execution engine.
If the CL is terminated while it waits for the EL (up to 30 s of retries), no
SIGTERM handler is installed yet, so the process dies with the store open and
without the quick-repair commit that redb performs on Drop. The next start then
runs a full repair: 1h04 on a 103 GB testnet store.

This moves open_store to just before State::builder, its first use, so the
store is not open during the EL wait. No behavior change on the success path.

Reproduce (v0.8.0, EL stopped):

1. systemctl start arc-malachite; wait for Database opened and the
   Failed to connect to Ethereum node retries.
2. systemctl stop arc-malachite during the retries.
3. Start the EL, then systemctl start arc-malachite:
   Database repair in progress: 0.00% instead of Database opened.

With this patch, step 1 logs no Opening database before the EL check, and
step 3 opens the store immediately and completes the handshake.

This is a minimal patch. The underlying issue is that a clean close depends on
the shutdown path: store.savepoint() is called explicitly on every exit path
(7 call sites across node.rs, app.rs, state.rs and coord_upgrade.rs) and
the SIGTERM handler is only installed after start() succeeds. A proper fix
would install the signal handler before the store is opened and funnel every
exit through a single teardown that stops the actors and closes the store, so
no path has to remember the savepoint.

[github-actions]

Hi @ZanCorDX,

Thank you for your interest in contributing to Arc Node.

This PR has been automatically closed because you are not assigned to issue #466. We require contributors to be explicitly assigned to an issue before submitting a PR.

To contribute properly:

1. Comment on issue CL: SIGTERM during startup while waiting for the EL forces a full redb repair on next start #466 requesting assignment
2. Wait for maintainer approval
3. Only submit a PR after you have been assigned

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