SizeSummary::tier_stats was populated for every scanned object but
apply_scanner_size_summary dropped it, so per-tier usage never reached
DataUsageInfo. Wire it through the same merge chain repl_target_stats
already uses, up to DataUsageInfo::tier_stats.
DataUsageEntry used the derived MessagePack encoding, which serialises
structs as arrays: appending a field turns the whole cache into a decode
error for older readers, so mixed-version nodes would invalidate each
other's cache every scan cycle. Give it the same hand-written
map-encoded Serialize DataUsageCacheInfo already carries, and record the
invariant in AGENTS.md.
Widen TierStats counters from i32 to u64 so a tier past 2^31 versions
cannot make checked_merge reject an entire usage snapshot, and drop the
duplicate TierStats/AllTierStats definitions in the scanner crate in
favour of the data-usage ones.
