fix: implement group commit collection window (500µs or 32KB)

Replace polling loop (Collect + Sleep) with proper batch collection:
- Block on first request via channel receive
- Start 500µs timer for collection window
- Accumulate requests until timer fires or batch reaches 32KB
- Shutdown handling at all blocking points

Design doc §3.2.5: '等待组提交触发（500µs 或 32KB，先到者触发）'

Add GroupCommitDelay config field (default 500µs, must be > 0 and < 10ms).

wal/writer.go

@@ -6,8 +6,8 @@ import (
 	"sync/atomic"
 	"time"
 
-	"github.com/dailz/go-kv/errkit"
 	"github.com/dailz/go-kv/config"
+	"github.com/dailz/go-kv/errkit"
 	"github.com/dailz/go-kv/memtable"
 )
 
@@ -101,24 +101,67 @@ func NewWalWriter(cfg *config.WalConfig, dir string, startSegmentID uint64, star
 
 func (ww *WalWriter) runLoop() {
 	defer ww.wg.Done()
+	const batchSizeThreshold = 32 * 1024
+
 	for {
 		select {
 		case <-ww.done:
 			ww.processRemaining()
 			return
-		default:
-		}
+		case req, ok := <-ww.queue.ch:
+			if !ok {
+				ww.processRemaining()
+				return
+			}
 
-		requests := ww.queue.Collect()
-		if len(requests) == 0 {
-			time.Sleep(100 * time.Microsecond)
-			continue
-		}
+			requests := []*CommitRequest{req}
+			accumulatedSize := estimateRequestSize(req)
+			timer := time.NewTimer(ww.cfg.GroupCommitDelay)
 
-		ww.processBatch(requests)
+		collect:
+			for accumulatedSize < batchSizeThreshold {
+				select {
+				case <-timer.C:
+					break collect
+				case req, ok := <-ww.queue.ch:
+					if !ok {
+						break collect
+					}
+					requests = append(requests, req)
+					accumulatedSize += estimateRequestSize(req)
+				case <-ww.done:
+					if !timer.Stop() {
+						select {
+						case <-timer.C:
+						default:
+						}
+					}
+					ww.sendError(requests, errkit.ErrWriteStopped)
+					ww.processRemaining()
+					return
+				}
+			}
+
+			if !timer.Stop() {
+				select {
+				case <-timer.C:
+				default:
+				}
+			}
+
+			ww.processBatch(requests)
+		}
 	}
 }
 
+func estimateRequestSize(req *CommitRequest) int {
+	size := 0
+	for _, entry := range req.Entries {
+		size += len(entry.Key) + len(entry.Value) + 4
+	}
+	return size
+}
+
 func (ww *WalWriter) processRemaining() {
 	for {
 		requests := ww.queue.Collect()