Expose native filesystem sources through public loading API

source.go

@@ -2,14 +2,31 @@ package configkit
 
 import (
 	"io"
+	"io/fs"
 
 	"gitea.maximumdirect.net/eric/ConfigKit/internal/engine"
 )
 
-// Source is an opaque configuration input. Construct sources with File, Bytes,
-// Reader, or NoSource; the zero value is invalid.
+// Source is an opaque configuration input. Construct sources with File,
+// Filesystem.File, Bytes, Reader, or NoSource; the zero value is invalid.
 type Source struct{ value engine.Source }
 
+// Filesystem is a reusable opaque namespace for a caller-owned fs.FS. Copies
+// preserve identity; separately constructed values are distinct even if their
+// labels or underlying filesystems match. The filesystem is never closed.
+type Filesystem struct{ value engine.Filesystem }
+
+// NewFilesystem borrows filesystem without opening it. Label must be nonempty
+// and safe to display; a nil filesystem or invalid label is reported when a
+// source from the returned namespace is consumed.
+func NewFilesystem(label string, filesystem fs.FS) Filesystem {
+	return Filesystem{value: engine.NewFilesystem(label, filesystem)}
+}
+
+// File selects a non-root fs.ValidPath name in this filesystem. Each load
+// reopens the file; constructor problems are reported when the source is used.
+func (f Filesystem) File(name string) Source { return Source{value: f.value.File(name)} }
+
 // File returns a source that reopens path for every load attempt.
 func File(path string) Source { return Source{value: engine.File(path)} }