fix(onboarding): History rung no longer reads a stale scan cache under a git-refs primary

The import-scan cache fingerprint only watched the v1 metadata branch tip, but under a git-refs checkpoint primary an import writes per-checkpoint refs (refs/entire/checkpoints/...) and never moves that branch. The post-import re-check then served the stale pre-import cache entry, so enable printed 'Imported 2 turn(s)' directly above '✗ History ... not imported'.

Fold an order-independent digest of the per-checkpoint refs into the fingerprint; a ref-iteration failure marks the scan uncacheable so a partial digest can never produce a false cache hit.