refactor(concurrency): remove zero-caller facade modules, fix feature build (#4530)

refactor(concurrency): remove zero-caller facade modules and fix no-default-features build (backlog#1025)

The audit in rustfs/backlog#1010 (consistent with #805) established that most of crates/concurrency was a decorative facade with zero production callers; the real runtime concurrency control lives in rustfs/src/storage/*. This deletes the dead facades and keeps only what the workspace actually consumes.

Deleted (zero callers verified by workspace-wide grep):
- manager.rs: ConcurrencyManager, lifecycle start/stop, misleading 'started' lifecycle logs
- config.rs: ConcurrencyConfig, ConcurrencyFeatures, from_env
- timeout.rs: TimeoutManager, TimeoutGuard, TimeoutManagerPolicy
- lock.rs: LockManager, LockScopeGuard, OptimizedLockGuard
- scheduler.rs: SchedulerManager, SchedulerPolicy, IoStrategy
- deadlock.rs facade: DeadlockManager, RequestTracker
- backpressure.rs facade: BackpressureManager, BackpressurePipe
- the prelude module, unused io-core re-exports, and all feature flags

Kept (real callers in ecstore/heal/rustfs):
- workload.rs admission contract types (unchanged)
- workers.rs Workers pool (unchanged, retained per #4498)
- GetObjectQueueSnapshot (moved from manager.rs to new queue.rs)
- PipeBackpressurePolicy (used by rustfs/src/storage/backpressure.rs)
- DeadlockMonitorPolicy (used by rustfs/src/storage/deadlock_detector.rs)
- OperationProgress re-export (used by rustfs/src/storage/timeout_wrapper.rs)

Removing the feature flags fixes the previously broken cargo check -p rustfs-concurrency --no-default-features (E0432). Docs and the logging guardrail file list are updated to match.

Ref: rustfs/backlog#1025

crates/concurrency/src/queue.rs

@@ -0,0 +1,84 @@
+// Copyright 2024 RustFS Team
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Disk permit queue snapshot for GetObject orchestration.
+
+/// Snapshot of disk permit queue usage for GetObject orchestration.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct GetObjectQueueSnapshot {
+    /// Total permits configured for disk reads.
+    pub total_permits: usize,
+    /// Permits currently in use.
+    pub permits_in_use: usize,
+}
+
+impl GetObjectQueueSnapshot {
+    /// Create a queue snapshot from total and available permits.
+    pub fn from_available_permits(total_permits: usize, available_permits: usize) -> Self {
+        Self {
+            total_permits,
+            permits_in_use: total_permits.saturating_sub(available_permits),
+        }
+    }
+
+    /// Return currently available permits.
+    pub fn permits_available(&self) -> usize {
+        self.total_permits.saturating_sub(self.permits_in_use)
+    }
+
+    /// Return queue utilization percentage in the 0-100 range.
+    pub fn utilization_percent(&self) -> f64 {
+        if self.total_permits == 0 {
+            0.0
+        } else {
+            (self.permits_in_use as f64 / self.total_permits as f64) * 100.0
+        }
+    }
+
+    /// Return whether the queue is considered congested.
+    pub fn is_congested(&self, threshold_percent: f64) -> bool {
+        self.utilization_percent() > threshold_percent
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_queue_snapshot() {
+        let snapshot = GetObjectQueueSnapshot::from_available_permits(64, 16);
+        assert_eq!(snapshot.permits_in_use, 48);
+        assert_eq!(snapshot.permits_available(), 16);
+        assert!(snapshot.is_congested(70.0));
+    }
+
+    #[test]
+    fn test_queue_snapshot_clamps_over_available_permits() {
+        let snapshot = GetObjectQueueSnapshot::from_available_permits(64, 96);
+        assert_eq!(snapshot.permits_in_use, 0);
+        assert_eq!(snapshot.permits_available(), 64);
+        assert_eq!(snapshot.utilization_percent(), 0.0);
+        assert!(!snapshot.is_congested(0.0));
+    }
+
+    #[test]
+    fn test_queue_snapshot_handles_zero_total_permits() {
+        let snapshot = GetObjectQueueSnapshot::from_available_permits(0, 0);
+        assert_eq!(snapshot.permits_in_use, 0);
+        assert_eq!(snapshot.permits_available(), 0);
+        assert_eq!(snapshot.utilization_percent(), 0.0);
+        assert!(!snapshot.is_congested(0.0));
+    }
+}