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§7.1 now records the 4 de-risking rungs (mockups → throwaway feasibility spike → vertical slice → full build), riskiest-API-assumption-first, so any forced design change lands on throwaway code. Adds coauthoring-feasibility- spike.md: 5 experiments (E1 editable proposals+Keep/Undo, E2 baseline router, E3 preview authorship, E4 comment loop, E5 discoverability) with PASS/FAIL criteria; closes open Q2/Q4/Q7 before any plan is written. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>