ui: probe-all-files walk and honest file tree coloring

- pysimFsLoadChildren now treats {success:false,error} (and any error
  payload) as a failed listing: absent DFs/ADFs render as a red cross
  without an expand arrow instead of silently opening empty; the
  /api/tree 500 body also carries exists:false for shape consistency
  with /api/select
- an expanded directory with zero children shows a gray (empty)
  placeholder
- new Probe all files button (data-needs=card) in the tree header:
  walks the whole model tree from MF, verifies every DF/ADF via
  /api/tree and every file (incl. custom entries) via /api/select,
  skips subtrees of absent dirs, shows N/total progress, toggles to
  Stop, and reports present/absent counts with elapsed time; results
  colour the current tree only
- tests: fs_load (error/retry/empty), fs_render (red cross, (empty)),
  fs_probe (flags, absent-dir skip, custom files, stop, summary);
  html structural check; help EN/RU, READMEs, AGENTS updated;
  SW cache v115 -> v116.

frontend/help.html

@@ -322,6 +322,7 @@
 		<li><strong>Read</strong> — reads the selected file (auto-detects transparent vs record files)</li>
 		<li><strong>Edit</strong> — modify hex data, <strong>Save</strong> to write back (or <strong>Cancel</strong>)</li>
 		<li><strong>Raw / Decoded</strong> — toggle between hex dump and pySim-decoded JSON</li>
+		<li><strong>Probe all files</strong> — walks the whole tree (including custom files) and marks every entry present (normal) or absent (red ✗, no expand arrow); empty-but-present DFs show <code class="font-mono text-sm">(empty)</code>. Shows progress <em>N / total</em>, can be stopped, and finishes with a present/absent summary. Files are only verified when expanded or probed — browsing stays lazy.</li>
 	</ul>
 
 	<h3 id="pysim-cmdline" class="text-lg font-medium mb-2">4.2 pySim command line</h3>