Hierarchical, durable provision citations

# Context

`Provision.citation` is a flat string (e.g., `"16(1)(a)"`) with no structural relationship to other provisions. Two consequences:

- **No hierarchy.** Queries like "all provisions under section 16" require parsing the citation string; there is no parent / child relation between a section, its subsections, and its paragraphs.
- **No durable identity.** When source legislation is amended, sections are renumbered — a flat string that names `"16(1)(a)"` today may point to a different provision after the next revision, with no migration path. This also blocks interoperability with other legal-text systems (regulators, citators), which use standardised identifier schemes.

# Outcome

Provision citations capture the section / subsection / paragraph hierarchy as a queryable relation and use a stable identifier scheme that survives renumbering and aligns with established standards (e.g., Akoma Ntoso or ELI).

# Acceptance criteria

- Provisions can be retrieved by parent (section / subsection / paragraph) without parsing citation strings.
- A provision's identifier remains stable when its source legislation is amended or renumbered.
- Citation identifiers follow a documented, recognised legal-citation scheme.
- Currently seeded provisions migrate to the new structure without data loss.