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

tests/unit/compact-cells.test.ts

@@ -299,8 +299,9 @@ describe("review round: cells the model may write", () => {
         expect(outcome.xml).toContain(
             '<mxCell id="3" value="B" vertex="1" parent="L1"',
         )
-        expect(foldCells(file, "L1")).toContain(
-            '<mxCell id="2" value="Start" style="rounded=1;" x="40" y="40" w="120" h="60"/>',
+        // Folding keeps a parent other than the default layer "1"
+        expect(foldCells(file)).toContain(
+            '<mxCell id="2" value="Start" style="rounded=1;" parent="L1" x="40" y="40" w="120" h="60"/>',
         )
     })