Pin strict UK spine parity to the contract share band #773

--strict is documented as the swap-acceptance posture, but any
--share-band in [0, 1) is accepted
(tools/verify_uk_spine_parity.py:468-504). A fresh probe used an empty
register, strict=true, band 0.9, and a −0.776937 water-share delta; the
result was verdict=parity, strict_failure=false, and no unsigned
differences.

Acceptance criteria:

• Make strict acceptance refuse a non-contract band, or isolate overrides in a
  diagnostic mode that cannot emit an acceptance verdict.
• Bind the effective band into the receipt and add an adversarial widened-band
  regression.

From the post-merge verification of #747 (round-1 review + fresh reproductions on merged main); full evidence: https://github.com/PolicyEngine/microcosm/blob/review/pr-747-audit/REVIEW-747.md

🤖 Filed by the microcosm owner session with Claude Code