Regenerate UK spine acceptance for the exact final 25-stage candidate #771

Current main has 25 production stages ending in age_tail
(tools/build_uk_frs_spine.py:93-119,845-853), while the committed L0 receipt
still states “all 24 stages”
(experiments/686-uk-spine-swap-receipts.md:330-342). The referenced receipt
under data/ukds/acceptance/686-spine-swap/ is not tracked, and the Markdown
evidence predates the value-changing final stage and SPI persisted-zero
changes. No post-round-1 commit refreshed it.

Acceptance criteria:

• Rebuild the exact final production composition after the E6/NHS repair.
• Commit disclosure-safe evidence binding candidate content SHA, stage roster
  and composition, twin-payload identity, E4–E8, and strict parity outcomes.
• Make the receipt fail if its stage roster or candidate identity differs from
  the production plan.

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