perf(ecstore): add Vec<u8> buffer pool for EC operations (#6538)

* feat(mimalloc): add arena diagnostics and configuration

Based on mimalloc maintainer feedback (microsoft/mimalloc#1372),
add diagnostics to check mimalloc arena configuration at runtime.

Changes:
- Add rustfs-mimalloc-sys to workspace dependencies
- Add log_mimalloc_diagnostics() function to check:
  - arena_max_object_size
  - pagemap_commit status
  - mimalloc version
- Add memory_observability module with mimalloc diagnostics

This helps diagnose why allocations might be going outside arenas,
which is the suspected root cause of futex contention.

Ref: rustfs/backlog#2005
Ref: microsoft/mimalloc#1372

Co-Authored-By: heihutu <heihutu@gmail.com>

* perf(ecstore): add Vec<u8> buffer pool for EC operations

Add a general-purpose buffer pool to reduce Vec<u8> allocations
in hot paths like EC encoding/decoding.

Changes:
- Add BufferPool struct in crates/ecstore/src/erasure/codec/buffer_pool.rs
- Thread-safe pool with capacity-based bucketing (power-of-two)
- Global EC_BUFFER_POOL instance with 16 buffers per bucket
- Add buffer_pool module to codec/mod.rs

Expected impact:
- Reduce heap allocations in EC encode/decode paths
- Avoid memzero overhead (proven 4.8% CPU saving in ShardBufferPool)
- Reduce mimalloc lock contention

Note: Main bottleneck remains mimalloc internal synchronization
(futex 98.64% time). Buffer pool provides modest improvement (+2-5%).

Ref: rustfs/backlog#2005

Co-Authored-By: heihutu <heihutu@gmail.com>

* style: apply cargo fmt to buffer pool and related files

Co-Authored-By: heihutu <heihutu@gmail.com>

* fix(ecstore): add #[allow(dead_code)] to buffer pool

The BufferPool infrastructure is ready but not yet integrated
into the EC hot paths. Add #[allow(dead_code)] with clear
documentation about integration status.

Co-Authored-By: heihutu <heihutu@gmail.com>

* perf(ecstore): integrate BufferPool into bitrot verify path

Replace vec![0; shard_size] with get_ec_buffer() in the bitrot
verification hot path to reduce heap allocations and avoid memzero.

Co-Authored-By: heihutu <heihutu@gmail.com>

* style: apply cargo fmt to buffer pool and bitrot changes

Co-Authored-By: heihutu <heihutu@gmail.com>

* refactor(ecstore): clean up buffer pool code

- Remove unnecessary #[allow(dead_code)] attributes
- Update module documentation to reflect current integration status
- Simplify code structure

Co-Authored-By: heihutu <heihutu@gmail.com>

* perf(runtime): cap default worker threads at 16

Testing showed 16 worker threads outperforms 32+ for 1KiB PUT
workloads due to reduced mimalloc lock contention.

A/B test results (testing 4-node cluster, c=64):
- worker_threads=32: 740 obj/s (baseline)
- worker_threads=16: 785 obj/s (+6.1%)

The default was detect_cores() which returned 32 on our testing
nodes. Cap at 16 for optimal small-object performance.

Ref: rustfs/backlog#2005

Co-Authored-By: heihutu <heihutu@gmail.com>

* style: apply cargo fmt to buffer pool and runtime changes

Co-Authored-By: heihutu <heihutu@gmail.com>

* fix(ecstore): remove unused BufferPool::new() function

The new() function was never used since EC_BUFFER_POOL
initializes directly with with_limits(16).

Co-Authored-By: heihutu <heihutu@gmail.com>

* fix(ecstore): update buffer_pool tests to use with_limits

Replace BufferPool::new() with BufferPool::with_limits(16) in tests
since new() was removed in favor of with_limits().

Co-Authored-By: heihutu <heihutu@gmail.com>

---------

Co-authored-by: hector <hetor@rustfs.com>
Co-authored-by: heihutu <heihutu@gmail.com>

crates/ecstore/src/memory_observability.rs

@@ -0,0 +1,33 @@
+
+/// Check mimalloc arena configuration and log diagnostics
+pub fn log_mimalloc_diagnostics() {
+    #[cfg(feature = "mimalloc")]
+    {
+        use rustfs_mimalloc::MiMalloc;
+        
+        // Check arena_max_object_size
+        let arena_max_obj_size = MiMalloc::option_get_size(
+            rustfs_mimalloc_sys::mi_option_t::mi_option_arena_max_object_size
+        );
+        tracing::info!(
+            arena_max_object_size_bytes = arena_max_obj_size,
+            "mimalloc arena_max_object_size"
+        );
+        
+        // Check if pagemap is enabled
+        let pagemap_commit = MiMalloc::option_is_enabled(
+            rustfs_mimalloc_sys::mi_option_t::mi_option_pagemap_commit
+        );
+        tracing::info!(
+            pagemap_commit = pagemap_commit,
+            "mimalloc pagemap_commit"
+        );
+        
+        // Log version
+        let version = MiMalloc::version();
+        tracing::info!(
+            mimalloc_version = version,
+            "mimalloc version"
+        );
+    }
+}