Unusable DATA_DIR error names DATA_DIR; report file number is atomic.Int64 (closes #148, closes #161)

When the backend runs on its own, a DATA_DIR that cannot be created or
read stopped the start with an error that named the path but not
DATA_DIR. Both errors now read DATA_DIR "<value>": <reason>, the form
config.New uses for the other settings. A test starts the buffer with
DATA_DIR below a regular file and checks the error.

The report file sequence counter is atomic.Int64 instead of
atomic.Uint64, as the Go style guide asks for int types. File names are
unchanged.

Model: opus-5-5

internal/reportbuf/reportbuf.go

@@ -88,7 +88,7 @@ type Buffer struct {
 	now func() time.Time
 	// seq numbers the report files, so that two named in the same
 	// millisecond still get different names.
-	seq      atomic.Uint64
+	seq      atomic.Int64
 	stopOnce sync.Once
 	// usedBytes is what Append checks against maxBytes: the size
 	// of the report files in dataDir, plus the reports waiting to
@@ -127,16 +127,18 @@ func New(
 
 	lc.Append(fx.Hook{
 		OnStart: func(_ context.Context) error {
+			// The errors name DATA_DIR, as config.New's do for the
+			// settings it checks.
 			err := os.MkdirAll(b.dataDir, dirPerms)
 			if err != nil {
-				return fmt.Errorf("create data dir: %w", err)
+				return fmt.Errorf("DATA_DIR %q: %w", b.dataDir, err)
 			}
 
 			// Report files left by earlier runs count too, and are
 			// the first to be deleted to make room.
 			files, err := reportFiles(b.dataDir)
 			if err != nil {
-				return err
+				return fmt.Errorf("DATA_DIR %q: %w", b.dataDir, err)
 			}
 
 			b.mu.Lock()