refactor(data-usage): own SizeSummary once, with the scanner's semantics (#6237)

`SizeSummary` and `ReplTargetSizeSummary` existed in both `rustfs-data-usage` and `rustfs-scanner`, and the two copies had drifted three ways: four size fields were `usize` in one and `i64` in the other, only the scanner's carried `tier_stats`, and — the difference that matters — the scanner's `add` saturated while the data-usage copy used plain `+=`, which panics on overflow in a debug build and wraps in a release one.

The data-usage copy is now the only definition and takes the scanner's shape and semantics, since that is the side a test already pinned (`MAX + 1 == MAX`). An equivalent saturation test now guards it in its new home. The scanner re-exports both types alongside the ones it already re-exported.

`DataUsageEntry::add_sizes` and `BucketUsageInfo::add_size_summary` are removed. Both took a `SizeSummary` and had no callers anywhere — they were the duplicate fold paths, and `apply_scanner_size_summary` is now the only one.

`actions_accounting` stays in the scanner as the `ScannerSizeSummaryExt` extension trait: it needs `ObjectInfo`, which sits above `rustfs-data-usage`, and an inherent impl on a foreign type is not allowed. The three call sites are unchanged.

Refs backlog#1828

crates/scanner/src/scanner_folder.rs

@@ -21,7 +21,7 @@ use std::time::{Duration, Instant, SystemTime};
 use crate::ReplTargetSizeSummary;
 use crate::data_usage_define::{
     DATA_USAGE_SCAN_CHECKPOINT_VERSION, DataUsageCache, DataUsageEntry, DataUsageHash, DataUsageHashMap, DataUsageScanCheckpoint,
-    DataUsageScanCheckpointReason, PendingScannerHeal, PendingScannerHealKind, SizeSummary, hash_path,
+    DataUsageScanCheckpointReason, PendingScannerHeal, PendingScannerHealKind, ScannerSizeSummaryExt, SizeSummary, hash_path,
 };
 use crate::error::ScannerError;
 use crate::runtime_config::{