fix(fs): absolute paths pass through Fs.path unprefixed on an unrestricted medium

fs.go

@@ -129,15 +129,21 @@ func (m *Fs) path(p string) string {
 		return PathJoin(cwd, p)
 	}
 
+	// An unrestricted medium passes an absolute path through as it
+	// stands — only a relative one was rewritten above. No "/" prefix
+	// here: prefixing was a no-op for a POSIX absolute path ("//x"
+	// cleans to "/x") but corrupted a Windows drive-letter one —
+	// filepath.Clean(`/C:\models`) is `\C:\models`, a path no Win32
+	// open resolves, which silently emptied every List/Read walk on
+	// an absolute path there.
+	if root == "/" {
+		return CleanPath(p, string(PathSeparator))
+	}
+
 	// Use a leading slash to resolve all .. and . internally
 	// before joining with the root. This is a standard way to sandbox paths.
 	clean := CleanPath("/"+p, string(PathSeparator))
 
-	// If root is "/", allow absolute paths through
-	if root == "/" {
-		return clean
-	}
-
 	// Strip leading "/" so Join works correctly with root
 	return PathJoin(root, clean[1:])
 }

fs_internal_test.go

@@ -20,6 +20,15 @@ func TestFs_Fs_path_Ugly(t *T) {
 	fsys := (&Fs{}).New("/")
 
 	AssertEqual(t, PathJoin(cwd.Value.(string), "relative.txt"), fsys.path("relative.txt"))
+
+	// An absolute path on an unrestricted medium comes back as itself —
+	// no "/" prefix bolted on first. The prefix was invisible on POSIX
+	// ("//x" cleans to "/x") and turned a Windows drive-letter path into
+	// `\C:\models`, which no Win32 open resolves. PathIsAbs recognises
+	// the drive-letter form on every platform, so this pins the contract
+	// everywhere, not just on a windows runner.
+	AssertEqual(t, `C:\models\gemma`, fsys.path(`C:\models\gemma`))
+	AssertEqual(t, CleanPath("/tmp//x/../y", string(PathSeparator)), fsys.path("/tmp//x/../y"))
 }
 func TestFs_Fs_validatePath_Good(t *T) {
 	root := t.TempDir()