fix: constrain account facts source binding rotation

[Pigbibi]

Summary

• Reuse the existing protected account-facts binding workflow for one existing IBKR source rotation.
• Preserve account identity and all other binding fields; accept only target and previous/next source identifiers from a protected secret.
• Preserve the existing Schwab binding flow.

Validation

• Actual embedded workflow logic: 9 synthetic cases passed.
• actionlint and diff check passed.
• Independent static review passed.

No live account data, secret values, native account identifiers, or private deployment mapping is included. This change does not trigger broker operations or change trading authorization.