Scoped general exemptions

# Context

`Provision.is_general_exemption` is a boolean flag. When set, a provision is treated as exempting every requirement in the legislation. In practice, general exemptions are scoped — they apply to specific sections under specific conditions, not to the entire act. The boolean cannot express that scope, so curators either over-apply the exemption (producing incorrect verdicts) or work around it by promoting it to per-requirement modifiers (which loses the "general" character).

# Outcome

A provision marked as a general exemption can declare which sections of the legislation it applies to, and under what conditions, while remaining identifiable as a general exemption rather than a per-requirement modifier.

# Acceptance criteria

- The scope of a general exemption (which provisions / sections it applies to) is recorded explicitly.
- A general exemption's scope can be queried without manual review of legal text.
- Compliance evaluation only considers a general exemption when the requirement under evaluation falls within its declared scope.
- Existing `is_general_exemption` data migrates to the new model without behavioural regression.