Make VaultID conditional on `LoanBrokerSet

[Tapanito]

High Level Overview of Change

LoanBrokerSet is a dual-purpose transaction — it creates a new LoanBroker when LoanBrokerID is absent, and modifies an existing one when LoanBrokerID is present. Previously VaultID was marked as Required unconditionally, but VaultID is meaningless (and potentially confusing) on update because the Vault association is fixed at creation time and cannot be changed.

• VaultID is required when creating a new LoanBroker (LoanBrokerID absent)
• VaultID must not be present when modifying an existing LoanBroker (LoanBrokerID present) — temINVALID if it is

Context of Change

Type of Change

• New XLS Draft
• XLS Update (changes to an existing XLS)
• XLS Status Change (e.g., Draft → Final, Draft → Stagnant)
• Process/Meta (changes to CONTRIBUTING.md, XLS-1, templates, etc.)
• Infrastructure (CI, workflows, scripts, website)
• Documentation (README updates, typo fixes)