ui: named FCI block holds the metadata; hide the empty content pane (v2.7.13)

- pysimFsInfoHtml puts the FID/type/size line inside the fieldset, so the
  selected file is one bordered block with its symbolic name in the border;
  files without an FCI keep the plain borderless line.
- The content pane starts hidden and is synced after every mutation
  (selection clears it, read success shows it, read failure keeps it hidden,
  edit/cancel re-sync): no empty box before a read or edit.
- tests: FCI ordering assertions; new fs_read.test.js (visibility helper,
  read success/failure/throw, markup + call sites); fs_edit extraction.

frontend/tests/fs_edit.test.js

@@ -27,7 +27,8 @@ let code = 'var pysimFsDecodedMode = false;\n'
 	+ 'var pysimFsSelected = null;\n'
 	+ 'var pysimFsTreeRoot = null;\n';
 for (const fn of ['pysimFsSetMode', 'pysimFsPillsEnabled', 'pysimFsEdit',
-	'pysimFsCancel', 'pysimFsResetEdit', 'pysimFsFindNode', 'pysimFsHasDecoder']) {
+	'pysimFsCancel', 'pysimFsResetEdit', 'pysimFsFindNode', 'pysimFsHasDecoder',
+	'pysimFsSyncContentVisibility']) {
 	code += extractFunc(html, fn) + '\n';
 }
 eval(code);