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/object-capacity/src/capacity_manager.rs

@@ -427,7 +427,6 @@ pub enum DataSource {
     /// Write triggered
     WriteTriggered,
     /// Fallback value
-    #[allow(dead_code)]
     Fallback,
 }
 
@@ -603,7 +602,6 @@ impl WriteRecord {
 
 /// Hybrid strategy configuration
 #[derive(Debug, Clone)]
-#[allow(dead_code)]
 pub struct HybridStrategyConfig {
     /// Scheduled update interval
     pub scheduled_update_interval: Duration,
@@ -998,14 +996,12 @@ impl HybridCapacityManager {
     }
 
     /// Get cache age
-    #[allow(dead_code)]
     pub async fn get_cache_age(&self) -> Option<Duration> {
         let cache = self.cache.read().await;
         cache.as_ref().map(|c| c.last_update.elapsed())
     }
 
     /// Get write frequency (writes/minute)
-    #[allow(dead_code)]
     pub async fn get_write_frequency(&self) -> usize {
         let record = &self.write_record;
         record.recent_write_count(record.monotonic_second())
@@ -1300,7 +1296,6 @@ pub fn get_capacity_manager() -> Arc<HybridCapacityManager> {
 ///     .update_capacity(CapacityUpdate::exact(1000, 0), DataSource::RealTime)
 ///     .await;
 /// ```
-#[allow(dead_code)]
 pub fn create_isolated_manager(config: HybridStrategyConfig) -> Arc<HybridCapacityManager> {
     Arc::new(HybridCapacityManager::new(config))
 }