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/persistence.test.ts

@@ -3,6 +3,7 @@
  */
 
 import {
+    chmodSync,
     existsSync,
     mkdtempSync,
     readdirSync,
@@ -84,6 +85,23 @@ describe("Autosaver", () => {
         expect(new Autosaver(null).load("mcp-back")).toBeNull()
     })
 
+    it("never replaces a saved file it could not read", () => {
+        const saver = new Autosaver(tempDir(), 10)
+        saver.schedule("mcp-locked", DIAGRAM)
+        saver.flush()
+        const path = saver.pathFor("mcp-locked") as string
+        chmodSync(path, 0o000)
+        try {
+            expect(saver.load("mcp-locked")).toBeNull()
+            // The blank page shown instead must not overwrite the diagram
+            saver.schedule("mcp-locked", BLANK)
+            saver.flush()
+        } finally {
+            chmodSync(path, 0o644)
+        }
+        expect(readFileSync(path, "utf-8")).toBe(DIAGRAM)
+    })
+
     it("does nothing when saving is off", () => {
         const saver = new Autosaver(null)
         expect(saver.pathFor("mcp-x")).toBeNull()