fix(mcp-server): recover sessions in one place, and more fixes from the third review

- A session whose state expired was recovered from its auto-save file only
  for the preview page; the tools built on their older copy and then
  overwrote the file. They now recover it first (restoreSavedSession).
- A preview tab that missed the last AI write pushed its older diagram
  over the recovered one after a restart. It now shows the recovered
  diagram and keeps its own copy in History.
- An empty record of what the model has seen (after load_diagram or a
  page tool on unseen changes) no longer lets one page of a multi-page
  document count for all, and get_diagram counts a page only once found.
- History: a thumbnail goes only to the entry it shows, the cached image
  never belongs to an older diagram, a re-serialized copy adds no entry,
  and a cleared document with its own pages is kept before a restore.
- The root cell id check reads attributes one by one: rack-id="1" or id
  text inside a label no longer counts.
- A compressed page counts as having cells; a saved file that could not
  be read is never written over.

packages/mcp-server/tests/edit-gate.test.ts

@@ -156,9 +156,15 @@ describe("markPageSeen", () => {
         expect(checkEditGate(marked, live).ok).toBe(false)
     })
 
-    it("counts everything as seen when the model saw nothing before", () => {
-        // It has no old copy of the other pages to edit from
+    it("does not count other pages when the record of what was seen is empty", () => {
+        // Empty also after load_diagram or a page tool on unseen changes,
+        // when the model may still remember an older copy of the pages
         const live = doc("a1", "b1")
+        expect(markPageSeen("", live, { page_id: "A" })).toBe("")
+    })
+
+    it("counts a one-page document as seen from its only page", () => {
+        const live = `<mxfile>${page("A", "a1")}</mxfile>`
         expect(markPageSeen("", live, { page_id: "A" })).toBe(live)
     })
 })