UK compute campaign, run 5: PE-UK static LSR counterparts to OBR's 19 supply-side scorings (#55 step 2–3)

The #55 harvest landed (#75): 266 OBR policy-effect claims, including all 19 per-measure supply-side scorings from Briefing Paper No.10 (% potential GDP, by channel, SB2023→SS2025). This issue computes the first PE-side counterparts — the cheapest of the #55 counterpart families, exercising machinery that already exists (PE-UK's labour supply responses) rather than building any.

Steps

• Register the engine vocabulary: policyengine-uk-lsr@<version> as an attributable PE-side engine in the results shape (the #55 plan's step 2), so LSR rows never masquerade as static rows.
• Map the 19 BP10 measures to PE reforms: most are the same measures already in the #56 OBR registry and #66 reckoner registry — reuse those specs; triage the rest (childcare, pension measures) honestly.
• Managed runs with LSR enabled: emit hours/FTE-equivalent and earnings responses per measure; convert to OBR's % potential GDP basis only via a documented, tested bridge (participation + hours channels separately, matching BP10's channel split).
• Descriptive comparison per measure × channel: OBR scoring vs PE-UK LSR, named axes (elasticity provenance, potential-vs-actual GDP basis, horizon), different_model class.
• The gate the #55 plan set: where static LSR cannot answer a claim (dynamic capital/GE channels), record it as the justification queue for OG-UK's plumbing — that decision record is a deliverable, not a footnote.

Depends on #75 (merged harvest) and a managed session; sequences after #76 so the LSR toggle rides a verified environment.