fix(scanner): surface per-tier usage in the data-usage snapshot (#5623)

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.

AGENTS.md

@@ -347,6 +347,11 @@ cargo run -p rustfs-filemeta --example dump_fileinfo -- "/path/to/file/xl.meta"
   absent, empty, and nil all mean "no value", never `Uuid::nil()`.
 - A remote-tier version of `None`/`""` means the tier bucket is unversioned:
   send **no** `versionId` on tier GET/DELETE.
+- Structs persisted in the scanner data-usage cache (`DataUsageCacheInfo`,
+  `DataUsageEntry`) carry a hand-written map-encoded `Serialize`. MessagePack
+  encodes derived structs as arrays, where an appended field makes the whole
+  cache a decode error for older readers — keep new fields `#[serde(default)]`
+  and keep the map encoding rather than reverting to `derive(Serialize)`.
 
 ## Naming Conventions