The update banner is driven by update_available from
/api/llama/update-status. Prebuilt (marker) installs decide this via
freshness.is_behind, which is mix-aware: a release at the same upstream
base build but with a new -mix-<sha> suffix counts as behind.
Source-build installs went through _source_build_status, which compared
only the numeric base build (installed_build < latest_build). When a new
prebuilt shared the installed upstream base (our usual mix re-tag at the
same base), it returned update_available=False and the banner never
showed. This is the common macOS case, where a failed prebuilt fetch
falls back to a source build that lacks the mix patches.
Make the source-build path mix-aware to match the marker path: same base
plus a -mix-<sha> tag now offers the update (and displays the mix tag),
a bare same-base rebuild does not, and the downgrade guard and unknown
-version fallback are unchanged.
bd9324b634