feat: add scheduler workload admission contracts (#3602)

docs/architecture/workload-admission-contracts.md

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+# Workload Admission Contracts
+
+This document records the `rustfs/backlog#660` PR-05 and PR-07 scheduler
+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:
+
+- 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.
+
+## Workload Class Contract
+
+`WorkloadClass` defines the required future admission categories:
+
+- Foreground read.
+- Foreground write.
+- Metadata.
+- Scanner.
+- Repair.
+- Replication.
+
+`AdmissionState`, `WorkloadAdmissionSnapshot`, and
+`WorkloadAdmissionRegistrySnapshot` define read-only status shapes for later
+runtime owners. They do not replace the current scheduler, request guard,
+scanner, heal, replication, or ECStore placement behavior.
+
+## Boundary Rules
+
+- `rustfs-concurrency` owns this reusable contract surface.
+- The contract does not depend on `rustfs-ecstore` or RustFS binary runtime
+  state.
+- No scheduler decision logic, queue capacity, Tokio runtime default, scanner
+  admission, heal admission, replication admission, placement, membership, or
+  NUMA behavior changes are part of this slice.