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).

config/config.go

@@ -4,14 +4,14 @@ package config
 import (
 	"fmt"
 	"math"
+	"time"
 )
 
 // WAL format constants derived from the binary layout specification.
 const (
-	walFileHeaderSize       uint64 = 32
-	walBlockSize            uint32 = 32 * 1024 // 32KB
+	walFileHeaderSize        uint64 = 32
 	physicalRecordHeaderSize uint64 = 7
-	walBatchHeaderSize      uint64 = 18 // flags(2) + baseSequence(8) + entryCount(4) + entriesSize(4)
+	walBatchHeaderSize       uint64 = 18 // flags(2) + baseSequence(8) + entryCount(4) + entriesSize(4)
 )
 
 // WalConfig holds configuration for the Write-Ahead Log subsystem.
@@ -38,6 +38,10 @@ type WalConfig struct {
 	// Default: 4MB.
 	MaxBatchSize uint32
 
+	// GroupCommitDelay is the maximum time to collect requests before writing a
+	// WAL group commit batch. Default: 500µs. Must be > 0 and < 10ms.
+	GroupCommitDelay time.Duration
+
 	// MaxKeyBytes is the maximum size of a single key in bytes.
 	// Default: 4KB.
 	MaxKeyBytes uint32
@@ -60,12 +64,13 @@ type WalConfig struct {
 func Defaults() WalConfig {
 	return WalConfig{
 		MaxSegmentSize:    64 * 1024 * 1024, // 64MB
-		BlockSize:         32 * 1024,         // 32KB
+		BlockSize:         32 * 1024,        // 32KB
 		SyncMode:          "always",
 		MaxBatchEntries:   10000,
 		MaxBatchSize:      4 * 1024 * 1024, // 4MB
-		MaxKeyBytes:       4 * 1024,        // 4KB
-		MaxInlineValue:    4 * 1024,        // 4KB
+		GroupCommitDelay:  500 * time.Microsecond,
+		MaxKeyBytes:       4 * 1024,         // 4KB
+		MaxInlineValue:    4 * 1024,         // 4KB
 		MemTableSize:      64 * 1024 * 1024, // 64MB
 		MaxImmutableCount: 3,
 	}
@@ -89,6 +94,9 @@ func (c *WalConfig) applyDefaults() {
 	if c.MaxBatchSize == 0 {
 		c.MaxBatchSize = d.MaxBatchSize
 	}
+	if c.GroupCommitDelay == 0 {
+		c.GroupCommitDelay = d.GroupCommitDelay
+	}
 	if c.MaxKeyBytes == 0 {
 		c.MaxKeyBytes = d.MaxKeyBytes
 	}
@@ -121,6 +129,10 @@ func (c *WalConfig) Validate() error {
 		return fmt.Errorf("config: MaxImmutableCount must be >= 1, got %d", c.MaxImmutableCount)
 	}
 
+	if c.GroupCommitDelay <= 0 || c.GroupCommitDelay >= 10*time.Millisecond {
+		return fmt.Errorf("config: GroupCommitDelay must be > 0 and < 10ms, got %s", c.GroupCommitDelay)
+	}
+
 	// --- Checked arithmetic invariant validation ---
 	// Mirrors the derivation in docs/design.md § WAL Segment Rotation.