refactor(mcp-server): make the diagram modules usable from the web app

The web app will reuse the MCP server's XML engine instead of its own
copy in lib/utils.ts, so these modules now run in the browser too.

- Relative imports end in .ts, rewritten to .js by tsc
  (rewriteRelativeImportExtensions); Next.js resolves them directly
- Every module uses the global DOMParser/XMLSerializer: native in the
  browser, linkedom in Node via installDomPolyfill. pages.ts parsed with
  linkedom but serialized with the global serializer, which throws in
  the browser
- The saxes syntax check moves to xml-syntax.ts, so the browser does not
  pull in linkedom; it now also rejects undeclared prefixes such as
  xlink:, as the browser does
- Page decompression uses pako and atob instead of node:zlib and Buffer
- hasCells moves to pages.ts, away from the file system code
- The duplicate cell id check counts UserObject/object ids
- wrapCellsInModel drops comments and text before the first cell, which
  the web app accepts today
- validateAndFixXml takes { strict: false } for diagrams with user content
- Web tests run these modules with a browser DOM (jsdom)
- saxes becomes a direct dependency of the web app

packages/mcp-server/package-lock.json

@@ -12,6 +12,7 @@
                 "@modelcontextprotocol/sdk": "^1.31.0",
                 "linkedom": "^0.18.0",
                 "open": "^11.0.0",
+                "pako": "^2.2.0",
                 "saxes": "^6.0.0",
                 "zod": "^4.0.0"
             },
@@ -20,6 +21,7 @@
             },
             "devDependencies": {
                 "@types/node": "^24.0.0",
+                "@types/pako": "^2.0.4",
                 "tsx": "^4.19.0",
                 "typescript": "^5",
                 "vitest": "^4.1.8"
@@ -909,6 +911,13 @@
                 "undici-types": ">=7.24.0 <7.24.7"
             }
         },
+        "node_modules/@types/pako": {
+            "version": "2.0.4",
+            "resolved": "https://registry.npmjs.org/@types/pako/-/pako-2.0.4.tgz",
+            "integrity": "sha512-VWDCbrLeVXJM9fihYodcLiIv0ku+AlOa/TQ1SvYOaBuyrSKgEcro95LJyIsJ4vSo6BXIxOKxiJAat04CmST9Fw==",
+            "dev": true,
+            "license": "MIT"
+        },
         "node_modules/@vitest/expect": {
             "version": "4.1.11",
             "resolved": "https://registry.npmjs.org/@vitest/expect/-/expect-4.1.11.tgz",
@@ -2601,6 +2610,22 @@
                 "url": "https://github.com/sponsors/sindresorhus"
             }
         },
+        "node_modules/pako": {
+            "version": "2.2.0",
+            "resolved": "https://registry.npmjs.org/pako/-/pako-2.2.0.tgz",
+            "integrity": "sha512-zJq6RP/5q+TO2OpFV3FHzlPnFjmkb7Nc99a5SNjJE+uu/PkpChs+NIZSSzbBoD+6kjiISXjfYdwj1ZRQ81dz/w==",
+            "funding": [
+                {
+                    "type": "github",
+                    "url": "https://github.com/sponsors/puzrin"
+                },
+                {
+                    "type": "github",
+                    "url": "https://github.com/sponsors/nodeca"
+                }
+            ],
+            "license": "(MIT AND Zlib)"
+        },
         "node_modules/parseurl": {
             "version": "1.3.3",
             "resolved": "https://registry.npmjs.org/parseurl/-/parseurl-1.3.3.tgz",