chore: adjudicate 26 bare dead_code allows across five crates (#6187)

Remove every bare `#[allow(dead_code)]` in io-core, object-capacity, targets, rio, and scanner. Each allow was stripped first and clippy was then asked which ones the compiler actually missed, so the verdicts rest on the diagnostic rather than on inspection.

23 were inert: they sat on `pub fn`s inside `pub mod`s, where `dead_code` does not apply, or on scanner integration-test helpers that the tests in the same file do call.

The remaining 3 are in rio's private `compress_index` module and the code behind them is deleted rather than annotated. `remove_index_headers` is dead and also wrong — after skipping the 4-byte chunk header it matches against `S2_INDEX_TRAILER` where `S2_INDEX_HEADER` sits, so it returns `None` for every well-formed index; rio-v2 carries the correct equivalent that is actually in use. `restore_index_headers` is its unreachable counterpart, likewise duplicated live in rio-v2. `Index::reset` is a private method with no caller.

Refs backlog#1823

crates/scanner/tests/lifecycle_integration_test.rs

@@ -206,7 +206,6 @@ async fn setup_isolated_test_env(init_expiry: bool) -> (Vec<PathBuf>, Arc<ECStor
 }
 
 /// Test helper: Create a test bucket
-#[allow(dead_code)]
 async fn create_test_bucket(ecstore: &Arc<ECStore>, bucket_name: &str) {
     (**ecstore)
         .make_bucket(bucket_name, &Default::default())
@@ -251,7 +250,6 @@ async fn modeled_versioned_delete_opts(bucket: &str, object: &str) -> ObjectOpti
 }
 
 /// Test helper: Set bucket lifecycle configuration
-#[allow(dead_code)]
 async fn set_bucket_lifecycle(bucket_name: &str) -> Result<(), Box<dyn std::error::Error>> {
     // Create a simple lifecycle configuration XML with 0 days expiry for immediate testing
     let lifecycle_xml = r#"<?xml version="1.0" encoding="UTF-8"?>
@@ -274,7 +272,6 @@ async fn set_bucket_lifecycle(bucket_name: &str) -> Result<(), Box<dyn std::erro
 }
 
 /// Test helper: Set bucket lifecycle configuration
-#[allow(dead_code)]
 async fn set_bucket_lifecycle_deletemarker(bucket_name: &str) -> Result<(), Box<dyn std::error::Error>> {
     // Create lifecycle rule that targets delete-marker cleanup only.
     // Keep Expiration.Days unset to avoid expiring live transitioned object versions.
@@ -297,7 +294,6 @@ async fn set_bucket_lifecycle_deletemarker(bucket_name: &str) -> Result<(), Box<
     Ok(())
 }
 
-#[allow(dead_code)]
 async fn set_bucket_lifecycle_delmarker_expiration(bucket_name: &str, days: i64) -> Result<(), Box<dyn std::error::Error>> {
     let lifecycle_xml = format!(
         r#"<?xml version="1.0" encoding="UTF-8"?>
@@ -320,7 +316,6 @@ async fn set_bucket_lifecycle_delmarker_expiration(bucket_name: &str, days: i64)
     Ok(())
 }
 
-#[allow(dead_code)]
 async fn set_bucket_lifecycle_transition_with_tier(
     bucket_name: &str,
     storage_class: &str,
@@ -368,7 +363,6 @@ async fn object_exists(ecstore: &Arc<ECStore>, bucket: &str, object: &str) -> bo
 }
 
 /// Test helper: Check if object exists
-#[allow(dead_code)]
 async fn object_is_delete_marker(ecstore: &Arc<ECStore>, bucket: &str, object: &str) -> bool {
     if let Ok(oi) = (**ecstore).get_object_info(bucket, object, &ObjectOptions::default()).await {
         println!("oi: {oi:?}");
@@ -379,7 +373,6 @@ async fn object_is_delete_marker(ecstore: &Arc<ECStore>, bucket: &str, object: &
     }
 }
 
-#[allow(dead_code)]
 async fn wait_for_object_absence(ecstore: &Arc<ECStore>, bucket: &str, object: &str, timeout: Duration) -> bool {
     let deadline = tokio::time::Instant::now() + timeout;