fix: prevent send-on-closed-channel panic in concurrent Put/Delete + Close (C7)

Put/Delete checked writeStopped then called queue.Submit (which sends on
a channel). Close set writeStopped then closed the channel. With no
synchronization between the check and Submit, a concurrent Close could
close the channel during the window, causing "send on closed channel"
panic in Put/Delete.

The CommitQueue.Close comment said "Callers must stop submitting before
Close" but Put/Delete didn't enforce this.

Changes:
- wal/writer.go: add submitMu sync.RWMutex. Put/Delete construct entry
  outside the lock, then hold RLock during writeStopped check + Submit;
  release before <-req.Result. Close holds write lock during
  writeStopped.Store + queue.Close, guaranteeing no Submit is in progress
  when the channel is closed.
- wal/writer_test.go: add TestConcurrentPutCloseNoPanic and
  TestConcurrentDeleteCloseNoPanic. 100 goroutines + 1 closer, recover
  panics. Run with -count=50 for regression detection.

Verified: all existing tests pass. New concurrent tests pass with
-count=50 (0 panics). go test -race ./... green.

Audit context: docs/audit-3.2.md C7.

wal/writer.go

@@ -58,6 +58,7 @@ type WalWriter struct {
 	seqManager   *SequenceManager
 	memTables    *MemTableList
 	writeStopped atomic.Bool
+	submitMu     sync.RWMutex // C7 fix: protects Submit vs Close critical section
 	done         chan struct{}
 	wg           sync.WaitGroup
 	closeOnce    sync.Once
@@ -311,31 +312,43 @@ func (ww *WalWriter) sendSuccess(requests []*CommitRequest, baseSequence uint64,
 
 // Put stores key with value.
 func (ww *WalWriter) Put(key, value []byte) error {
-	if ww.writeStopped.Load() {
-		return errkit.ErrWriteStopped
-	}
-
-	req := ww.queue.Submit([]*WalEntry{{
+	entry := &WalEntry{
 		OpType:    OpPut,
 		ValueKind: VKInline,
 		Key:       cloneBytes(key),
 		Value:     cloneBytes(value),
-	}})
+	}
+
+	// C7 fix: RLock protects check + Submit. Close holds write lock during
+	// writeStopped.Store + queue.Close, so Submit cannot send on a closed channel.
+	ww.submitMu.RLock()
+	if ww.writeStopped.Load() {
+		ww.submitMu.RUnlock()
+		return errkit.ErrWriteStopped
+	}
+	req := ww.queue.Submit([]*WalEntry{entry})
+	ww.submitMu.RUnlock()
+
 	result := <-req.Result
 	return result.Err
 }
 
 // Delete removes key.
 func (ww *WalWriter) Delete(key []byte) error {
-	if ww.writeStopped.Load() {
-		return errkit.ErrWriteStopped
-	}
-
-	req := ww.queue.Submit([]*WalEntry{{
+	entry := &WalEntry{
 		OpType:    OpDelete,
 		ValueKind: VKNone,
 		Key:       cloneBytes(key),
-	}})
+	}
+
+	ww.submitMu.RLock()
+	if ww.writeStopped.Load() {
+		ww.submitMu.RUnlock()
+		return errkit.ErrWriteStopped
+	}
+	req := ww.queue.Submit([]*WalEntry{entry})
+	ww.submitMu.RUnlock()
+
 	result := <-req.Result
 	return result.Err
 }
@@ -368,8 +381,14 @@ func (ww *WalWriter) IsWriteStopped() bool {
 // Close drains queued writes, stops the writer goroutine, syncs, and closes the segment.
 func (ww *WalWriter) Close() error {
 	ww.closeOnce.Do(func() {
+		// C7 fix: write lock waits for all in-flight Put/Delete Submits to
+		// complete. While we hold this lock, no Submit can be in progress,
+		// so queue.Close is safe (no send-on-closed-channel panic).
+		ww.submitMu.Lock()
 		ww.writeStopped.Store(true)
 		ww.queue.Close()
+		ww.submitMu.Unlock()
+
 		close(ww.done)
 		ww.wg.Wait()
 		if err := ww.segManager.Sync(); err != nil {