docs(design): prose-tests programme design log

[leeovery]

Summary

• Design log for the prose-tests programme — the missing test layer: natural-language cases an agent executes by walking the real skills against materialised, engine-built project states.
• Contract P1–P7: structured NL cases, worlds built by fixture recipes + golden snapshots (never hand-typed, never imagined), deterministic perimeter in CI (rebuild-compare + anchor staleness, zero tokens), walker/grader separation, on-command runs with diff-scoped selection, Sonnet→Opus escalation, happy path as first-class corpus.
• Stack root — the framework, happy-path corpus (one PR per work type), and rounds-4–6 failure harvest stack on top; the log accrues an entry per landed PR and merges at the end.

Test plan

• Docs-only; no behaviour to test. CI checks arrive with stage 2.

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