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.

docs/architecture/global-state-inventory.md

@@ -107,7 +107,7 @@ inventory. Generic function-local names such as `CACHE`, `LOCK`, `INIT`, and
 | `AUTH_FS` | `rustfs/src/storage/access.rs` | Cache or constant / owner-local cache | Authorization tag-condition lookup keeps its filesystem helper private to the access owner. |
 | `DEADLOCK_DETECTOR` | `rustfs/src/storage/deadlock_detector.rs` | Process-global owner-local state | Deadlock detector lifecycle state stays private to the storage deadlock detector owner. |
 | `CONCURRENCY_MANAGER`, `ACTIVE_GET_REQUESTS`, `ACTIVE_PUT_REQUESTS` | `rustfs/src/storage/concurrency/*` | Process-global owner-local scheduler state | Storage concurrency manager and request counters remain inside the storage concurrency owner boundary. |
-| `GET_OBJECT_BUFFER_THRESHOLD_WARNED`, `GET_READER_STREAM_BUFFER_SIZE_OVERRIDE`, function-local `ENABLED`, `OBJECT_SEEK_SUPPORT_THRESHOLD`, `OBJECT_SEEK_SUPPORT_CONCURRENCY_THRESHOLDS` | `rustfs/src/app/object_usecase.rs` | Cache or constant / owner-local cache | Object GET/seek tuning caches and warning guards stay private to object usecase helpers. |
+| `GET_OBJECT_BUFFER_THRESHOLD_WARNED`, `GET_READER_STREAM_BUFFER_SIZE_OVERRIDE`, function-local `ENABLED`, `OBJECT_SEEK_SUPPORT_THRESHOLD`, `OBJECT_SEEK_SUPPORT_CONCURRENCY_THRESHOLDS` | `rustfs/src/app/object/get.rs` | Cache or constant / owner-local cache | Object GET/seek tuning caches and warning guards stay private to object usecase helpers. |
 | `SUPPORTED_HEADERS` | `rustfs/src/storage/options.rs` | Cache or constant / owner-local constant | Supported-header lookup state stays private to storage option parsing. |
 | `AUDIT_TARGET_SPECS`, `NOTIFICATION_TARGET_SPECS` | `rustfs/src/admin/handlers/audit.rs`, `rustfs/src/admin/handlers/event.rs`, `rustfs/src/admin/handlers/plugins_instances.rs` | Cache or constant / owner-local constant | Admin target descriptor tables stay private to their handler owners. |
 | `SITE_REPLICATION_PEER_CLIENT` | `rustfs/src/admin/handlers/site_replication.rs` | Process-global owner-local cache | Site-replication peer client cache stays private to site-replication handlers. The state RMW transaction holds no process-local mutex — see `rustfs/src/admin/site_replication_state.rs`. |