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/src/persistence.ts

@@ -23,11 +23,18 @@ import { hasCells } from "./pages.ts"
 const DELAY_MS = 1000
 const MAX_FILES = 50
 
+/** Expand a leading ~ to the home directory (shells do this, MCP hosts don't). */
+export function expandHome(p: string): string {
+    if (p === "~") return homedir()
+    if (p.startsWith("~/") || p.startsWith("~\\")) return homedir() + p.slice(1)
+    return p
+}
+
 /** DRAWIO_DATA_DIR, default ~/.next-ai-drawio; "off" disables saving. */
 export function defaultDataDir(): string | null {
     const dir = process.env.DRAWIO_DATA_DIR
     if (dir === "off") return null
-    return dir || join(homedir(), ".next-ai-drawio")
+    return dir ? expandHome(dir) : join(homedir(), ".next-ai-drawio")
 }
 
 export class Autosaver {