fix: bind IBKR reconciliation to configured account

[Pigbibi]

Summary

• require the private IBKR expected-digest baseline to include the account-scope digest
• evaluate account identity against the configured digest instead of the current observation
• add a regression test for account substitution with otherwise matching state digests

Validation

• focused reconciliation tests: 9 passed
• full suite: 506 passed, 5 skipped
• ruff and diff checks passed

Safety

No broker request, runtime activation, deployment, order, cash, account, or baseline-variable change was performed.