RSI: validate curriculum proposals with executable fixtures

[w4ffl35]

Limitation\n\nRecurring failures can produce curriculum proposals with a ground-truth command, but proposals are currently untrusted records and are not automatically validated before reuse.\n\n## Proposed direction\n\nValidate generated tasks against checked-in fixtures or an evaluator-owned test corpus. Track proposal provenance, validation output, and promotion status.\n\n## Acceptance criteria\n\n- Invalid commands, missing fixtures, and non-reproducible tasks remain unvalidated.\n- A validated task can be replayed from a clean base commit.\n- Curriculum promotion is explicit and represented in the archive/report.