Provision-to-provision cross-references

# Context

Legal text frequently contains references between provisions: "subsection (1) does not apply where (7) applies", "for the purposes of section 3, see section 16(2)", and similar. The schema currently has no place to record these relationships. When a cross-reference matters for compliance evaluation, it is captured only indirectly — by attaching the modifying provision to each affected `Requirement` via `RequirementModifier`. The cross-reference is therefore re-derived by curators for every requirement it touches; the relation between the two provisions themselves is invisible to the database.

# Outcome

Cross-references between provisions are recorded as first-class relations on the `Provision` graph, queryable independently of any `Requirement` that uses them.

# Acceptance criteria

- A cross-reference between two provisions can be recorded once and retrieved when querying either side.
- Curating a `Requirement` that depends on an existing cross-reference does not require re-stating the relationship.
- The cross-reference type (e.g., exemption, redirection, definition reference) is recorded.