CIT check 2 — numeric regression and golden fixtures

[nikki-t]

Epic — CIT check 2: numeric regression + golden fixtures + --promote.

• Freeze the tiny-ROI bundle to Zenodo/DARUS; fetch it by reusing _copy_or_download_* from run-confluence-locally (dir_structure.py:97-223).
• Add cit/regression.py: per-variable rtol/atol, shape-first, stochastic bounds.
• Add fixtures/manifest.yml.
• Add the --promote PR flow: two-channel candidate/blessed, atomic rename + optimistic-concurrency pointer.

Deferred to a later phase (needs golden data). Sub-issues to be broken out when scheduled.