Legislation versioning and point-in-time queries

# Context

The compliance database holds only the current version of each provision, definition, and requirement. When source legislation is amended, old text is replaced; there is no way to retrieve the version that was in force at a given date. This blocks re-evaluation of historical verdicts (auditing whether a verdict was correct under the law at the time it was issued) and makes long-term reproducibility impossible.

# Outcome

Every versioned compliance entity (`Provision`, `Definition`, `Requirement`, and their modifiers) carries effective-date metadata, and the system can evaluate compliance against the state of the law as of any specified date.

# Acceptance criteria

- Each versioned entity records the date range for which it is in force.
- Past versions of a provision / definition / requirement are preserved when the law is amended; they are not overwritten.
- A compliance evaluation can be requested as of a specified date and returns a verdict consistent with the law in force on that date.
- A previously-issued verdict can be re-evaluated against the law in force at the time of issuance.