fix(mcp-server): keep both pages when get_diagram meets a page export, and more review fixes

Found by the PR review, each with a test that failed first:
- get_diagram during a page export returned the one-page projection on
  screen as the whole document (6 of 6 times when timed so). The preview
  page no longer answers a sync while a projection shows, and syncs after
  reloading, so the poll that restores the real document exports it.
- Exports are numbered on the server too: a late result of an export that
  timed out was saved as the next export's file.
- In Chrome, a new_xml with a syntax error counted the <parsererror>
  element as a second cell, so the web app rejected edits that auto-fix
  repairs ("must contain exactly one cell").
- hasCells missed single-quoted ids, so screenshot_diagram called such a
  diagram empty and auto-save never created its file.
- A literal \n directly under a <diagram> that has a model passed
  validation; only text-only pages are compressed data.
- A wrapped mxCell repeating its UserObject's id took the wrapper's place
  in edits, so delete and update left an empty or nested wrapper.
- Bare cells with a shape or edge id of "0" or "1" are rejected with a
  clear message instead of being renamed, which broke their edges.
- DRAWIO_DATA_DIR expands ~, which JSON configs pass on as it is.

packages/mcp-server/tests/http-server.test.ts

@@ -258,9 +258,28 @@ describe("export requests", () => {
         await postJson("/api/state", {
             sessionId: id,
             exportData: "data:image/png;base64,AAAA",
+            exportId: poll.exportId,
         })
         expect(getState(id)?.exportOptions).toBeUndefined()
     })
+
+    it("ignores a late result of an export that already timed out", async () => {
+        const id = "mcp-export-late"
+        setState(id, "<mxfile>x</mxfile>")
+        requestExport(id, "png")
+        const first = JSON.parse(
+            (await request(`/api/state?sessionId=${id}`)).body,
+        )
+        // The server gave up on the first export and asked for the next
+        requestExport(id, "svg")
+        await postJson("/api/state", {
+            sessionId: id,
+            exportData: "data:image/png;base64,LATE",
+            exportId: first.exportId,
+        })
+        expect(getState(id)?.exportData).toBeUndefined()
+        expect(getState(id)?.exportFormat).toBe("svg")
+    })
 })
 
 describe("preview page", () => {