refactor(rustfs): split object_usecase.rs into per-operation app/object modules (#6670)

* refactor(rustfs): carve app/object out of object_usecase.rs — shared, extract, test_support children (backlog#1841 step 1)

Mechanical move-only split of rustfs/src/app/object_usecase.rs (19.7K lines). The file body moves to rustfs/src/app/object/mod.rs, and the first self-contained slices move into children: shared.rs (cross-cutting helpers: quota admission, response checksum injection, object-lock write validation, table-catalog mutation guard, deadlock request guard, proxy passthrough utilities), extract.rs (snowball auto-extract path incl. tar/pax helpers and execute_put_object_extract), and cfg(test) test_support.rs for cross-module test scaffolding. object_usecase.rs stays as a thin pub use facade so every existing crate::app::object_usecase:: path keeps working.

No behavior change: items move verbatim; the only source edits are visibility widenings required by the new module boundaries (private -> pub(super); pub(super) -> pub(crate) for the three helpers multipart_usecase and the app gating tests import). Guard scripts that pinned rustfs/src/app/object_usecase.rs now scan the rustfs/src/app/object tree, and the table_catalog source-text guard test concatenates the split files.

* refactor(rustfs): move the GetObject read path into app/object/get.rs (backlog#1841 step 2)

Move-only continuation of the object_usecase split: cold-fill orchestration, disk-permit admission, streaming readers and resume control, stream-buffer tuning, execute_get_object / execute_get_object_attributes, the GET replication proxy helpers, and their unit tests move from app/object/mod.rs into app/object/get.rs. Items keep their original text; cross-module call sites rely on the visibility widenings introduced in step 1.

* refactor(rustfs): move the PutObject and CopyObject paths into app/object (backlog#1841 step 3)

Move-only continuation: put.rs takes the PUT body admission and timeout readers, zero-copy and eager-commit machinery, execute_put_object, and the PUT unit tests; copy.rs takes the copy namespace/lifecycle lock helpers and execute_copy_object with its tests. Two source edits beyond visibility widenings: PutObjectChecksums fields become pub(super) (read by shared::apply_trailing_checksums across the new module boundary) and one relative super::storage_api call in the copy path becomes crate::app::storage_api since super now resolves to app::object. The table_catalog source-text guard concatenates the new files.

* refactor(rustfs): finish the object_usecase split — delete, head, restore modules (backlog#1841 step 4)

Move-only completion: delete.rs takes the delete helpers, cfg(test) delete hooks, and execute_delete_object/execute_delete_objects; head.rs takes execute_head_object with the HEAD replication proxy helpers; restore.rs takes execute_restore_object. app/object/mod.rs is now just the shared import prelude, module wiring, and the DefaultObjectUsecase struct with its constructors, accessors, and the execute_select_object_content delegation; the emptied tests module is gone. The delete re-export glob is cfg(test)-gated because its only cross-module consumers are the delete test hooks.

The table_catalog source-text guard now isolates the delete entrypoints from app/object/delete.rs, and doc/comment references that pointed at rustfs/src/app/object_usecase.rs internals now point at the per-operation modules.

scripts/check_logging_guardrails.sh

@@ -1014,7 +1014,7 @@ if rg -n -U '(info|warn)!\(\s*target: "rustfs::heal::manager",[\s\S]{0,1000}"Hea
   exit 1
 fi
 
-if rg -n -U 'info!\([\s\S]{0,1000}"GetObject streaming body resumed from a reopened object read"' rustfs/src/app/object_usecase.rs >/dev/null; then
+if rg -n -U 'info!\([\s\S]{0,1000}"GetObject streaming body resumed from a reopened object read"' rustfs/src/app/object >/dev/null; then
   echo "❌ logging guardrail violation: successful per-object GetObject resume events must stay below INFO" >&2
   exit 1
 fi
@@ -1056,7 +1056,7 @@ trace_hot_spans=(
   "crates/ecstore/src/core/sets.rs:list_objects_v2"
   "crates/ecstore/src/set_disk/ops/list.rs:list_objects_v2"
   "rustfs/src/app/bucket_usecase.rs:execute_list_objects_v2"
-  "rustfs/src/app/object_usecase.rs:execute_get_object"
+  "rustfs/src/app/object:execute_get_object"
 )
 
 for hot_span in "${trace_hot_spans[@]}"; do