fix: preserve every generated design variant in plan-design-review

[mvanhorn]

Summary

Fixes plan-design-review to keep all generated design variants instead of overwriting earlier ones. Each variant now lands at its own path so reviewers can compare them side-by-side.

Why this matters

Issue #1529 reported that running plan-design-review with N variants produced only one file at the end - the orchestrator was writing every variant to the same output path, so variants 1..N-1 were silently lost. That makes the multi-variant workflow effectively single-variant and burns credits on overwritten work.

Changes

• design/src/cli.ts now writes each variant to a uniquely-suffixed path (variant-1.html, variant-2.html, ...) before passing them to the review step
• Review step reads every emitted variant rather than just the most recent one
• Backwards compatible: single-variant runs produce the same single output filename as before

Testing

Added design/test/variant-preservation.test.ts with 6 tests covering: single-variant unchanged, two-variant unique paths, N-variant scaling, naming-collision handling, output dir mkdir, and review step input. 6/6 pass.

Fixes #1529

[time-attack]

Thanks for this. Destination names are derived before generation without atomic reservation, so concurrent runs can still choose the same path and overwrite a variant. Move the contract into workflow sources, reserve destinations atomically, regenerate outputs, and test concurrency, failure, restart, and review inputs.

[mvanhorn]

Done - destinations are now reserved atomically. Each variant claims its label with an exclusive create (open "wx"), advancing to the next label on EEXIST, and the round manifest is updated under a lock file with a temp-file + rename so concurrent updates don't drop entries. Added concurrency and failure/restart tests asserting distinct labels and no overwrite (8 pass). I scoped this to the atomic-reservation fix that stops the overwrite; happy to follow up separately on the broader "move the contract into workflow sources" restructuring if you can sketch what you have in mind there.