fix(02): write fixtures exclusively and only on successful flush (WR-01)

Co-authored-by: Cursor <cursoragent@cursor.com>

tide/fixture.go

@@ -49,6 +49,15 @@ func ParseFlow(raw []byte) (Flow, error) {
 
 // SaveFlow writes a validated flow atomically, syncing before rename.
 func SaveFlow(path string, flow Flow) error {
+	return saveFlow(path, flow, false)
+}
+
+// SaveFlowExclusive writes a validated flow and fails if path already exists.
+func SaveFlowExclusive(path string, flow Flow) error {
+	return saveFlow(path, flow, true)
+}
+
+func saveFlow(path string, flow Flow, exclusive bool) error {
 	if err := validateFlow(flow); err != nil {
 		return err
 	}
@@ -62,6 +71,30 @@ func SaveFlow(path string, flow Flow) error {
 			return fmt.Errorf("tide: create fixture dir: %w", err)
 		}
 	}
+	if exclusive {
+		f, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0o644)
+		if err != nil {
+			return fmt.Errorf("tide: create fixture: %w", err)
+		}
+		ok := false
+		defer func() {
+			_ = f.Close()
+			if !ok {
+				_ = os.Remove(path)
+			}
+		}()
+		if _, err := f.Write(raw); err != nil {
+			return fmt.Errorf("tide: write fixture: %w", err)
+		}
+		if err := f.Sync(); err != nil {
+			return fmt.Errorf("tide: sync fixture: %w", err)
+		}
+		if err := f.Close(); err != nil {
+			return fmt.Errorf("tide: close fixture: %w", err)
+		}
+		ok = true
+		return nil
+	}
 	tmp, err := os.CreateTemp(dir, ".tide-*.tmp")
 	if err != nil {
 		return fmt.Errorf("tide: create temp fixture: %w", err)