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

tests/unit/mcp-core.test.ts

@@ -0,0 +1,90 @@
+/**
+ * The web app runs the MCP server's diagram modules in the browser, where
+ * DOMParser and XMLSerializer are the native ones (jsdom here), not the
+ * linkedom polyfill the MCP tests use.
+ */
+import { deflateRaw } from "pako"
+import { describe, expect, it } from "vitest"
+import { applyDiagramOperations } from "@/packages/mcp-server/src/diagram-operations.ts"
+import { editDiagram } from "@/packages/mcp-server/src/edit-diagram.ts"
+import { decompressPageContent } from "@/packages/mcp-server/src/load-diagram.ts"
+import {
+    hasCells,
+    normalizeToMxfile,
+    wrapCellsInModel,
+} from "@/packages/mcp-server/src/pages.ts"
+import { getXmlSyntaxError } from "@/packages/mcp-server/src/xml-syntax.ts"
+import { validateAndFixXml } from "@/packages/mcp-server/src/xml-validation.ts"
+
+const box = (id: string, parent = "1") =>
+    `<mxCell id="${id}" value="${id}" vertex="1" parent="${parent}"><mxGeometry x="0" y="0" width="80" height="40" as="geometry"/></mxCell>`
+const edge = (id: string, source: string, target: string) =>
+    `<mxCell id="${id}" edge="1" parent="1" source="${source}" target="${target}"><mxGeometry relative="1" as="geometry"/></mxCell>`
+const file = (cells: string) =>
+    normalizeToMxfile(wrapCellsInModel(cells), {
+        pageId: "p1",
+        pageName: "Page-1",
+    }) as string
+
+describe("MCP diagram modules with a browser DOM", () => {
+    it("wraps bare cells into a valid file", () => {
+        const xml = file(box("a") + box("b"))
+        expect(xml).toContain('<diagram id="p1" name="Page-1">')
+        expect(validateAndFixXml(xml).valid).toBe(true)
+        expect(hasCells(xml)).toBe(true)
+        expect(hasCells(file(""))).toBe(false)
+    })
+
+    it("fixes the case of a misspelled tag in model XML", () => {
+        const result = validateAndFixXml(
+            file(box("a"))
+                .replace('<mxCell id="a"', '<mxcell id="a"')
+                .replace("</mxCell></root>", "</mxcell></root>"),
+        )
+        expect(result.valid).toBe(true)
+        expect(result.fixes.join(" ")).toMatch(/tag case/)
+    })
+
+    it("reports syntax errors with line and column", () => {
+        expect(getXmlSyntaxError("<mxfile><diagram></mxfile>")).toMatch(
+            /^1:\d+/,
+        )
+        expect(getXmlSyntaxError(file(box("a")))).toBeNull()
+    })
+
+    it("deletes a cell with its edges", () => {
+        const xml = file(box("a") + box("b") + edge("e", "a", "b"))
+        const { result, errors } = applyDiagramOperations(xml, [
+            { operation: "delete", cell_id: "a" },
+        ])
+        expect(errors).toEqual([])
+        expect(result).not.toContain('id="a"')
+        expect(result).not.toContain('id="e"')
+        expect(result).toContain('id="b"')
+    })
+
+    it("runs a whole edit and serializes the target page", () => {
+        const outcome = editDiagram(
+            file(box("a")),
+            [{ operation: "add", cell_id: "b", new_xml: box("b") }],
+            {},
+        )
+        expect(outcome.ok).toBe(true)
+        if (outcome.ok) expect(outcome.xml).toContain('id="b"')
+
+        const failed = editDiagram(
+            file(box("a")),
+            [{ operation: "add", cell_id: "b", new_xml: box("b") + box("c") }],
+            {},
+        )
+        expect(failed.ok).toBe(false)
+    })
+
+    it("decompresses a draw.io compressed page", () => {
+        const model = wrapCellsInModel(box("a"))
+        const deflated = deflateRaw(encodeURIComponent(model))
+        const base64 = btoa(String.fromCharCode(...deflated))
+        expect(decompressPageContent(base64)).toBe(model)
+        expect(decompressPageContent("not compressed")).toBeNull()
+    })
+})