Replace the hand-maintained DATA_VERSION constant with a build-time SHA-256
hash of static/data/ embedded as a 'version' field in the preload manifest.
The runtime reads it via fetchWithFallback and clears the IDB cache when it
differs, so any data edit auto-invalidates cached data — no manual bump to
forget. The no-version (offline/stale) and IDB read-error guards are kept,
so the cache is never wiped on an unreadable signal.
Closes audit item L1.
catarrh