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/xml-validation.test.ts

@@ -308,3 +308,13 @@ describe("text between tags", () => {
         ).toBe(true)
     })
 })
+
+describe("text directly under a page", () => {
+    it("fixes a literal \\n before the model of a page", () => {
+        const r = validateAndFixXml(
+            `<mxfile><diagram id="p" name="P">\\n${model(`<mxCell id="2" vertex="1" parent="1"/>`)}</diagram></mxfile>`,
+        )
+        expect(r.valid).toBe(true)
+        expect(r.fixed).not.toContain("\\n")
+    })
+})