Issue keys whose comments have been fully fetched (phase-3 resume marker). Unique keys; order not significant.
Every cached issue, keyed by "owner/repo#number" (see issueKey).
External-blocker references discovered in phase 2/4 that still need fetching.
Which of the four scan phases this cache is currently in.
Per-target-repo resume state, keyed by "owner/repo".
On-disk schema version; bumped only on incompatible shape changes.
ISO timestamp this cache was last persisted by saveCache.
The whole cache: data store and resume journal in one object.