The post-install path cleared only the in-memory freshness caches and then
re-primed the 24h disk cache with a forced GitHub refresh. When that refresh
cannot reach GitHub, latest_published_release falls back to the last-good disk
value, so a still-fresh same-base mix tag cached before the swap (b9596-mix-aaa
vs the just-installed b9596-mix-bbb) is replayed and the prebuilt reads as
behind, surfacing a false update banner that points back at the build that was
just replaced.
Give reset_caches a drop_disk option and use it on the update path: with the
disk cache gone, an offline post-install refresh leaves latest as None and the
banner fails open (off) instead of lingering on the stale same-base value. The
no-arg form stays in-memory only. Adds regression coverage for the drop, the
default no-op, and the fail-open vs stale-replay contrast.
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