refactor(ecstore): move ObjectIO/ObjectOperations into set_disk::ops::object (backlog#821) (#4290)

P6 of the SetDisks God-Object split (tracking backlog#815, issue backlog#821;
depends on P5 #4288). Relocate the core object read/write hot-path contract
impls out of the set_disk/mod.rs God-Object into their own module home:

- impl ObjectIO for SetDisks (~1,010 lines) -> set_disk/ops/object.rs
- impl ObjectOperations for SetDisks (~1,137 lines) -> set_disk/ops/object.rs
- Registered pub(crate) mod object; under set_disk/ops.

Pure move — zero logic change. Both contracts stay implemented for SetDisks, so
their EcstoreObjectIO / EcstoreObjectOperations associated-type bounds are
unchanged and the contract-compat tests still guard them. Method bodies are
moved verbatim: a whitespace-insensitive token-stream diff of the two impl
blocks (with their #[async_trait::async_trait] attributes) against the pre-move
mod.rs source is byte-identical (5,754 tokens each) — NO visibility widening was
required, because the impls reach SetDisks helpers and the P5 io_primitives
through inherent self./Self:: calls that resolve across modules unchanged. The
two inherent impl SetDisks blocks that sat between the trait impls (lock batch
helpers) remain in mod.rs, verified present exactly once and uncorrupted.

The issue's borrow/Arc-clone-avoidance optimization is intentionally deferred:
it is a perf-sensitive change requiring the #738 benchmark and would risk the
'byte-level behavior unchanged' acceptance; this PR delivers the relocation.

Verification:
- cargo check / clippy -D warnings -p rustfs-ecstore --all-targets: clean
- cargo test -p rustfs-ecstore --lib: 1841 passed, 0 failed
- all five arch guard scripts: pass
- token-stream diff of moved impls vs original: identical (mod.rs diff is a pure
  relocation; ObjectIO/ObjectOperations each defined exactly once post-move)

crates/ecstore/src/set_disk/ops/mod.rs

@@ -21,3 +21,4 @@ pub(crate) mod bucket;
 pub(crate) mod heal;
 pub(crate) mod list;
 pub(crate) mod multipart;
+pub(crate) mod object;