chore: converge stale TODOs and apply safe fills (backlog#646) (#4322)

Second TODO-convergence round over the current tree (backlog#646). All
line numbers in the old inventory had gone stale after the set_disk /
diagnostics / cluster refactors, so this re-scans and reduces the marker
count from 144 to 99.

STALE removals (comment describes already-implemented behavior, or dead
commented-out blocks) across ecstore (set_disk ops/core, store,
cluster/rpc, bucket/metadata_sys, services), iam, filemeta, s3select and
rustfs auth/object_usecase. No behavior change.

Safe fills, each verified:
- filemeta: replication_info_equals now also compares
  replication_state_internal (function currently has no callers; adds a
  regression test).
- bitrot: drop the confirmed-unused `_want` parameter from bitrot_verify
  and the now-unused `sum` on LocalDisk::bitrot_verify, removing a
  Bytes::copy_from_slice allocation. Streaming verify uses the file's
  embedded per-shard hash, never the passed sum.
- signer: rename v4_ignored_headers -> V4_IGNORED_HEADERS and drop the
  non_upper_case_globals allow.
- admin/heal: test_decode was #[ignore]d and used serde_urlencoded on a
  JSON body (would panic); rewire to serde_json::from_slice to match the
  production decode path, add assertions, un-ignore.

Verified: cargo fmt; cargo check on touched crates; tests pass
(filemeta, signer, bitrot, heal::test_decode); arch guardrail scripts
pass.

crates/ecstore/src/erasure/coding/bitrot.rs

@@ -12,7 +12,6 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-use bytes::Bytes;
 use pin_project_lite::pin_project;
 use rustfs_utils::HashAlgorithm;
 use std::io::IoSlice;
@@ -221,7 +220,6 @@ pub async fn bitrot_verify<R: AsyncRead + Unpin + Send>(
     want_size: usize,
     part_size: usize,
     algo: HashAlgorithm,
-    _want: Bytes, // FIXME: useless parameter?
     mut shard_size: usize,
 ) -> std::io::Result<()> {
     let mut hash_buf = vec![0; algo.size()];