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

scripts/check_logging_guardrails.sh

@@ -54,8 +54,6 @@ checked_files=(
   "crates/scanner/src/scanner_io.rs"
   "crates/scanner/src/scanner_folder.rs"
   "crates/concurrency/src/workers.rs"
-  "crates/concurrency/src/manager.rs"
-  "crates/concurrency/src/lock.rs"
   "crates/concurrency/src/deadlock.rs"
   "crates/trusted-proxies/src/global.rs"
   "crates/trusted-proxies/src/config/loader.rs"