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# Repair XI VAT routing and canonicalization

An offline invoice-compliance pipeline uses the frozen python-stdnum source in
`/app/vendor/project`. Its EU VAT router and United Kingdom VAT validator do
not correctly handle the `XI` prefix used for Northern Ireland movements.

Repair the existing production modules:

- `/app/vendor/project/stdnum/eu/vat.py`
- `/app/vendor/project/stdnum/gb/vat.py`

Do not replace the library with a new validator or weaken its existing United
Kingdom rules. The repaired behavior must satisfy all of these requirements:

1. `XI` is a recognized EU VAT jurisdiction and dispatches to the existing GB
   VAT implementation. `GB` itself remains outside `MEMBER_STATES` and is not
   accepted by the EU router.
2. The GB implementation accepts either `GB` or `XI` as a leading prefix and
   removes exactly one such prefix during compaction. It must not strip an
   arbitrary pair of letters or repeatedly remove duplicate prefixes.
3. EU-layer compaction and validation preserve the canonical `XI` prefix,
   while direct GB-layer validation returns the prefix-free GB payload.
4. Existing 9/12-digit checksum rules and the `GD`/`HA` special-format
   component rules remain enforced. Invalid checksums, malformed payloads,
   duplicate prefixes, and invalid special-number ranges must stay invalid.

The deterministic workload is `/app/input/invoices.json`. After repairing the
source, run:

```bash
python3 /app/run_audit.py \
  --input /app/input/invoices.json \
  --output /app/output.json
```

The artifact must follow `/app/output_contract.json` with
`output_schema_version` equal to `vat-compliance-report.v1`. Include every
input row exactly once in input order. Do not modify the workload, contract,
runner, license, utility modules, or frozen upstream regression file.

Before finishing, run focused syntax and behavior checks against both repaired
modules and regenerate `/app/output.json` from the final source tree.