UK counterpart compute pipeline: managed policyengine-uk sims on the certified bundle

The US side computes counterparts via pipeline/compute_counterparts.py (managed_microsimulation on the certified Populace artifact). The UK side has only ad-hoc campaign runs. Build the standing UK equivalent so UK lanes can move ingested → computed repeatably.

• pipeline/compute_uk_counterparts.py (or a country-parametrized refactor of the existing pipeline) running policyengine-uk managed sims against the current certified populace-uk bundle, under the machine-wide sim lock
• Promote country from constant to key across the tidy schema and build_comparison.py, per the EUROMOD/UKMOD row in docs/ARCHITECTURE.md
• Emit weighted counts only on the PE side; rates/deltas stay derived in one place (build_comparison.py), matching the US contract
• Full provenance per row: engine_version, data_bundle, computed_at, run_id — same fields the campaign JSONL already carries
• Reproducing section in README for the UK path

Depends on the UK harvest ingest path; unblocks DWP take-up, HBAI, HMRC, OBR, UKMOD lanes (#3).

🤖 Generated with Claude Code

Start with pipeline/compute_counterparts.py, then read the EUROMOD/UKMOD row in docs/ARCHITECTURE.md and inspect build_comparison.py and the tidy schema. Check the existing campaign JSONL provenance and the certified populace-uk bundle path before running the UK harvest ingest. Done means repeatable locked UK computation, country-keyed comparison rows, complete provenance, and a README reproduction section.