refactor(scanner): split scanner_io.rs into child modules (#6294)

Split the 5369-line scanner_io.rs (39% inline tests) into a canonical
scanner_io.rs + scanner_io/ module tree with zero behavior change:

- scanner_io.rs (~660): constants, metadata-error constructors, the
  bucket scan plan, cycle-status classification helpers, the ScannerIO /
  ScannerIOCache / ScannerIODisk traits, and ScannerCycleResult
- scanner_io/dirty_usage.rs (~300): process-wide dirty-usage statics
  and the acknowledgment protocol
- scanner_io/guards.rs (~270): concurrency gauges and RAII guards
- scanner_io/cache.rs (~410): scanner cache locks and the snapshot
  persist/publish path
- scanner_io/io_cycle.rs (~390), io_cache.rs (~1160), io_disk.rs
  (~230): the ECStore / SetDisks / Disk trait implementations
- scanner_io/publish_gate_tests.rs (~750) and tests.rs (~1340): the two
  inline test modules as child modules

All crate paths are unchanged: the lib.rs scanner_io re-exports and
every crate::scanner_io:: consumer (scanner.rs, remote_scanner,
scanner_folder, and cross-crate rustfs users) resolve through root
re-exports with their original visibilities (pub stays pub, pub(crate)
stays pub(crate)). Cross-module items gain pub(super), whose scope
equals the old single-module privacy domain. Code is moved verbatim
apart from those markers, per-module import headers, and rustfmt
re-wraps.

The logging-guardrail nsscanner_disk skip-set_disks rule now points at
scanner_io/io_disk.rs where the function moved; the pattern and
thresholds are unchanged.

Co-authored-by: heihutu <heihutu@gmail.com>

scripts/check_logging_guardrails.sh

@@ -845,7 +845,9 @@ done
 # `set_disks` expands every Disk through Debug, including raw format bytes and
 # the full per-operation metrics ring. Keep it out of the INFO scanner span.
 scanner_disk_skip_pattern='#\[(tracing::)?instrument\([^]]*skip\([^)]*\bset_disks\b[^)]*\)[^]]*\)\][[:space:]]*async fn nsscanner_disk\b'
-if ! rg -U "$scanner_disk_skip_pattern" crates/scanner/src/scanner_io.rs >/dev/null; then
+# nsscanner_disk lives in the scanner_io/io_disk.rs child module since the
+# scanner_io module-tree split.
+if ! rg -U "$scanner_disk_skip_pattern" crates/scanner/src/scanner_io/io_disk.rs >/dev/null; then
   echo "❌ logging guardrail violation: nsscanner_disk must skip set_disks in its tracing instrumentation" >&2
   exit 1
 fi