feat(mcp-server): get_selection reads the cells the user selected in the shell

The server asks the preview tab for the selection the way it asks for an export (a random request id in GET /api/state, the answer in a POST with that id, 10 s to answer). The shell answers through the editor bridge with each cell's id, label, an edge's ends, a shape's geometry and the page on screen; without a same-origin editor it says so, and the tool names the external draw.io. The classic page cannot answer, so the tool says that at once. screenshot_diagram's description now says a page selector renders that page without changing the page on screen (PNG exports by pageId already did).

lib/drawio/editor-bridge.ts

@@ -11,6 +11,7 @@
  */
 import { isSameDocument, sameFileVars } from "@/lib/diagram-diff"
 import { hasCells } from "@/packages/mcp-server/src/pages.ts"
+import type { SelectionAnswer } from "@/packages/mcp-server/src/selection.ts"
 import { type SelectedCell, useCanvasStore } from "@/stores/canvas-store"
 
 type EditorUi = any
@@ -599,6 +600,54 @@ function readSelection(): SelectedCell[] {
         }))
 }
 
+/**
+ * The selection with what an editing model needs (the MCP's get_selection):
+ * the page it is on, each cell's label, an edge's ends, a shape's geometry
+ * as the XML has it (relative to its container) and that container. Null
+ * without an editor (a cross-origin draw.io).
+ */
+export function readSelectionDetails(): SelectionAnswer | null {
+    const g = graph()
+    if (!g) return null
+    const page = ui?.currentPage
+    const defaultParent = g.getDefaultParent?.()
+    const cells = (g.getSelectionCells() as any[])
+        .filter((cell) => cell?.id)
+        .map((cell) => {
+            const info: SelectionAnswer["cells"][number] = {
+                id: String(cell.id),
+                label: cellLabel(cell),
+                edge: !!g.model.isEdge(cell),
+            }
+            if (info.edge) {
+                const source = g.model.getTerminal(cell, true)
+                const target = g.model.getTerminal(cell, false)
+                if (source?.id) info.source = String(source.id)
+                if (target?.id) info.target = String(target.id)
+            } else {
+                const geo = g.getCellGeometry(cell)
+                if (geo) {
+                    info.geometry = {
+                        x: geo.x,
+                        y: geo.y,
+                        width: geo.width,
+                        height: geo.height,
+                    }
+                }
+            }
+            const parent = g.model.getParent(cell)
+            if (parent?.id && parent !== defaultParent) {
+                info.parent = String(parent.id)
+            }
+            return info
+        })
+    return {
+        pageId: page?.getId ? String(page.getId()) : null,
+        pageName: page?.getName ? String(page.getName()) : null,
+        cells,
+    }
+}
+
 function readSelectionRect() {
     const g = graph()
     if (!g || g.isSelectionEmpty()) return null