fix: strip nested BasePathFs base segment from BasePathFile.Name

BasePathFile.Name trimmed only filepath.Clean(path) from the wrapped
file name. filepath.Clean drops the trailing separator, and a nested
BasePathFile reports a name whose base segment is preceded by a leading
separator (for example "/123/x"), which the cleaned base ("123") does
not match. The base then leaked into Name and callers double-prepended
it. Trim the separator-prefixed form too so nested BasePathFs report a
name that round-trips.

Fixes #428

basepath.go

@@ -37,7 +37,16 @@ type BasePathFile struct {
 
 func (f *BasePathFile) Name() string {
 	sourcename := f.File.Name()
-	return strings.TrimPrefix(sourcename, filepath.Clean(f.path))
+	base := filepath.Clean(f.path)
+	name := strings.TrimPrefix(sourcename, base)
+	if name == sourcename {
+		// filepath.Clean drops the leading separator from the base path, so a
+		// name produced by a wrapped (nested) BasePathFile can still carry the
+		// base preceded by a separator. Strip that form as well, otherwise the
+		// base segment leaks into the reported name.
+		name = strings.TrimPrefix(sourcename, string(filepath.Separator)+base)
+	}
+	return name
 }
 
 func (f *BasePathFile) ReadDir(n int) ([]fs.DirEntry, error) {