Reduce resident account lookup overhead

[bmuddha]

Outcome

Reduce resident-account lookup overhead in AccountsDB and determine whether shared load metrics materially contribute to contention observed around ensure_accounts.

Scope

• Add a mode-only AccountsDB lookup for callers that need Option<AccountMode>. It should read volatile mode without cloning the OwnedAccount and read persisted mode without materializing a full account, while retaining the concurrent-publish sequence check.

Acceptance criteria

• AccountsDB exposes one mode-only lookup that handles volatile, persisted, absent, and concurrently published accounts correctly.
• The mode-only path avoids cloning or materializing a complete account solely to inspect its mode.