chore(ecstore): adjudicate 32 bare dead_code allows (#6173)

Replace every bare `#[allow(dead_code)]` in ecstore with either a deletion or a per-item allow carrying a `reason`. Blanket allows at module, struct, and impl level silence the lint for future members too, so each is narrowed to the members that are actually dead.

Delete the dead cluster in `config/heal.rs` (`Config`, its three methods, `RUSTFS_BITROT_CYCLE_IN_MONTHS`, `parse_bitrot_config`) rather than annotate it: it has no callers and is unreachable outside the crate, and `parse_bitrot_config` would panic on its disabled path via `Duration::from_secs_f64(-1.0)`. `DEFAULT_KVS` stays, since the config registry uses it.

Correct two `reason` strings on `Checksum::new` and `PutObjReader::md5_current_hex_string`, which are methods but carried a field-only rationale.

Refs backlog#1823

Co-authored-by: houseme <housemecn@gmail.com>

crates/ecstore/src/core/pools.rs

@@ -1996,11 +1996,11 @@ impl PoolMeta {
         Ok(false)
     }
 
-    #[allow(dead_code)]
     pub fn validate(&self, pools: Vec<Arc<Sets>>) -> Result<bool> {
         struct PoolInfo {
             position: usize,
             completed: bool,
+            #[allow(dead_code, reason = "written but never read back (backlog#1823)")]
             decom_started: bool,
         }
 
@@ -4958,13 +4958,19 @@ fn is_disk_online_state(state: &str) -> bool {
 }
 
 #[deprecated(since = "0.1.0", note = "Use fallback_total_capacity_dedup instead")]
-#[allow(dead_code)]
+#[allow(
+    dead_code,
+    reason = "superseded by the replacement named in the comment at pools.rs:5071 (backlog#1823)"
+)]
 fn fallback_total_capacity(disks: &[rustfs_madmin::Disk]) -> usize {
     fallback_total_capacity_dedup(disks)
 }
 
 #[deprecated(since = "0.1.0", note = "Use fallback_free_capacity_dedup instead")]
-#[allow(dead_code)]
+#[allow(
+    dead_code,
+    reason = "superseded by the replacement named in the comment at pools.rs:5071 (backlog#1823)"
+)]
 fn fallback_free_capacity(disks: &[rustfs_madmin::Disk]) -> usize {
     fallback_free_capacity_dedup(disks)
 }