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/bucket/metadata_sys.rs

@@ -295,7 +295,6 @@ impl BucketMetadataSys {
         let mut futures = Vec::new();
 
         for bucket in buckets.iter() {
-            // TODO: HealBucket
             let api = self.api.clone();
             let bucket = bucket.clone();
             futures.push(async move {
@@ -319,14 +318,13 @@ impl BucketMetadataSys {
 
         let mut mp = self.metadata_map.write().await;
 
-        // TODO:EventNotifier,BucketTargetSys
+        // TODO: EventNotifier
         for res in results {
             match res {
                 Ok(res) => {
                     if let Some(bucket) = buckets.get(idx) {
                         let x = Arc::new(res);
                         mp.insert(bucket.clone(), x.clone());
-                        // TODO:EventNotifier,BucketTargetSys
                         BucketTargetSys::get().set(bucket, &x).await;
                     }
                 }