fix(mcp-server): fix XSS and crashes, make XML validation strict

- Validate and escape the mcp session id; only serve localhost Host/Origin
- Malformed URLs and session ids return errors instead of crashing the process
- Strict XML syntax check with saxes (linkedom never reports parse errors)
- autoFixXml no longer corrupts valid XML; attribute newlines serialized as entities
- Sessions stay alive while polled; browser pushes carry a base version (409 on conflict)
- Page tools respect the edit gate; UTF-8 bodies decoded correctly
- Export replies matched to requests and serialized; xml sync export handled
- UserObject/object cells addressable by id; history restored by stable id; logs off stdout

packages/mcp-server/tests/edit-gate.test.ts

@@ -9,11 +9,11 @@
  * reads as a user edit.
  */
 
-import { DOMParser } from "linkedom"
 import { beforeAll, describe, expect, it } from "vitest"
+import { installDomPolyfill } from "../src/dom.js"
 
 beforeAll(() => {
-    ;(globalThis as any).DOMParser = DOMParser
+    installDomPolyfill()
 })
 
 import { checkEditGate, contentFingerprint } from "../src/edit-gate.js"