refactor(mcp-server): remove dead code and fold repeated tool plumbing

- index.ts: registerTool() does the session check, try/catch and the
  write queue for all 15 tools; fail()/ok(), syncFromBrowser(),
  commitDocument() and describePages() replace the copied blocks; drop
  pickPageSelector, currentSession.version, the input ?? {} defaults;
  add_page reuses prepareNewModel() split out of prepareNewDiagram
- xml-validation.ts: drop the hand-written syntax checks that saxes
  already reports, MAX_XML_SIZE and the stale header
- http-server.ts: one pendingExport object, no unread response fields,
  no CORS/OPTIONS branch for same-origin requests, fewer exports
- diagram-operations parseNewCell(), addHistory returns void,
  normalizeToMxfile without host, foldCells without layer, expandStyles
  removed, unused preview.css rule removed, no .d.ts output
- xml-examples.ts styleRules() shares the Styles paragraphs with the
  web prompt
- tests: setupFiles installs the DOM polyfill once, server-wiring uses
  the SDK client, mcp-core.test.ts drops the cases MCP tests already
  cover

packages/mcp-server/tests/wrap-cells.test.ts

@@ -3,13 +3,7 @@
  * a page, like in the web app, and the server adds the wrapper and root cells.
  */
 
-import { beforeAll, describe, expect, it } from "vitest"
-import { installDomPolyfill } from "../src/dom.ts"
-
-beforeAll(() => {
-    installDomPolyfill()
-})
-
+import { describe, expect, it } from "vitest"
 import { prepareNewDiagram, reservedIdError } from "../src/new-diagram.ts"
 import { hasCells, wrapCellsInModel } from "../src/pages.ts"
 import { validateAndFixXml } from "../src/xml-validation.ts"
@@ -100,6 +94,22 @@ describe("hasCells", () => {
             hasCells(`<mxCell id = "0"/><mxCell id = "1" parent="0"/>`),
         ).toBe(false)
     })
+
+    it("does not mistake ids that start with 0 or 1 for root cells", () => {
+        expect(
+            hasCells(
+                '<mxCell id="0"/><mxCell id="1" parent="0"/><mxCell id="10"/>',
+            ),
+        ).toBe(true)
+    })
+
+    it("counts a cell wrapped in a UserObject", () => {
+        expect(
+            hasCells(
+                '<mxCell id="0"/><mxCell id="1" parent="0"/><UserObject id="u" link="x"><mxCell vertex="1" parent="1"/></UserObject>',
+            ),
+        ).toBe(true)
+    })
 })
 
 describe("reservedIdError", () => {