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

docs/architecture/scheduler-baseline.md

@@ -9,9 +9,8 @@ runtime-builder ownership. It does not define new behavior.
 | Surface | Current owner | Current responsibility | Migration boundary |
 |---|---|---|---|
 | `ConcurrencyManager` | `rustfs/src/storage/concurrency/manager.rs` | Owns the RustFS S3 read-path disk-read semaphore, I/O metrics, priority queue, storage media detection, access-pattern detection, and buffer strategy. | Keep request admission and I/O metrics behavior stable until a controller can consume the same state explicitly. |
-| `SchedulerManager` | `crates/concurrency/src/scheduler.rs` | Provides a reusable facade over `rustfs-io-core::IoScheduler` and derives buffer/priority decisions from `SchedulerPolicy`. | Treat it as reusable library surface; do not assume the RustFS S3 read path has already moved to this facade. |
-| `BackpressureManager` | `crates/concurrency/src/backpressure.rs` | Provides a reusable duplex-pipe backpressure facade over `rustfs-io-core::BackpressureMonitor`. | Keep pipe sizing and watermark policy separate from object-read disk semaphore admission. |
-| RustFS backpressure monitor | `rustfs/src/storage/backpressure.rs` | Tracks object-pipe watermark state used by RustFS storage backpressure tests and helpers. | Preserve current state labels and watermark semantics when consolidating with reusable facades. |
+| I/O scheduler core | `crates/io-core/src/scheduler.rs` | Owns the reusable buffer-size and priority algorithms consumed by the RustFS S3 read path (`rustfs/src/storage/concurrency/io_schedule.rs`). The former `SchedulerManager` facade in `rustfs-concurrency` was removed as zero-caller dead code (backlog#1025). | Treat `rustfs-io-core` as the reusable algorithm surface; the RustFS S3 read path owns its own scheduling wiring. |
+| RustFS backpressure monitor | `rustfs/src/storage/backpressure.rs` | Tracks object-pipe watermark state used by RustFS storage backpressure tests and helpers, using the shared `PipeBackpressurePolicy` from `crates/concurrency/src/backpressure.rs`. The former `BackpressureManager`/`BackpressurePipe` facade in `rustfs-concurrency` was removed as zero-caller dead code (backlog#1025). | Preserve current state labels and watermark semantics; keep pipe sizing and watermark policy separate from object-read disk semaphore admission. |
 | `Workers` | `crates/concurrency/src/workers.rs` | Provides cooperative worker-slot admission with `take`, `give`, and `wait`; current background workflows use it for bounded set workers. | Preserve blocking/wakeup semantics and over-release clamping. |
 | Scanner cycle budget | `crates/scanner/src/scanner_budget.rs` | Cancels a child token when runtime, object-count, or directory-count budget is reached. | Preserve partial-cycle reason mapping and checkpoint accounting. |
 | Heal admission | `crates/heal/src/heal/manager.rs`, `crates/heal/src/heal/channel.rs`, `rustfs_common::heal_channel` | Owns priority queue admission, duplicate merge/drop/full results, active-task tracking, retry admission, and channel responses. | Preserve low-priority scanner behavior and high-priority escalation gates. |

docs/architecture/workload-admission-contracts.md

@@ -5,18 +5,18 @@ preservation and runtime workload-class contract slice.
 
 ## Preservation Coverage
 
-The `rustfs-concurrency` tests pin the current reusable scheduler and
-admission-facing behavior before later snapshot extraction:
+The `rustfs-concurrency` tests pin the current reusable admission-facing
+behavior before later snapshot extraction:
 
 - Worker slot over-release remains clamped by the configured worker limit.
-- Scheduler default buffer and priority thresholds remain unchanged.
-- Scheduler priority boundaries remain high below the high threshold, normal at
-  both thresholds, and low above the low threshold.
-- Backpressure pipe metadata reads preserve buffer capacity and state without
-  mutating the manager state.
 - `GetObjectQueueSnapshot` preserves saturated, over-available, and zero-total
   permit semantics.
 
+The former reusable scheduler and backpressure-pipe facades (and their
+preservation tests) were removed as zero-caller dead code in backlog#1025;
+scheduler buffer/priority behavior is now pinned by `rustfs-io-core` and
+`rustfs/src/storage/concurrency` tests.
+
 ## Workload Class Contract
 
 `WorkloadClass` defines the required future admission categories: