docs: propose conditional choices and decision rollback

[han4wluc]

Document the proposed conditional choices contract: per-item conditions, a required fallback, selection validation, and rollback to the decision before selection effects. Link the proposal from the existing rollback contract.

The design covers save compatibility, localization, implementation sequencing, and required future verification. This PR contains only docs/ConditionalChoices.md and docs/Rollback.md; the feature remains a proposal with no runtime changes.

Validation: Prettier passes for both documents, all relative Markdown links resolve, and git diff --cached --check passes.