fix(mcp-server): review fixes for get_selection

Server (src/http-server.ts, src/index.ts, src/selection.ts,
src/new-diagram.ts):
- overlapping get_selection calls take turns (readSelection, one slot per
  session as the export slot) instead of replacing each other's request,
  which left one of them with a false "tab not in front" timeout
- an answer is taken only while a request is pending (both ids undefined
  compared equal)
- the tool sees the page start_session actually opens: with dist/shell
  missing the classic page is in use, which never answers, so the tool
  says so instead of timing out
- the result lists at most 100 cells and counts the rest: a whole large
  diagram selected would fill the model's context
- every <diagram> the model sends without an id gets one, so the page id
  the shell reports exists in the server's document

Shell (shell/mcp-sync-core.ts, shell/use-mcp-sync.ts,
contexts/diagram-context.tsx, lib/drawio/editor-bridge.ts):
- an answer whose POST failed is sent again at the next poll
- no answer while a full load has yet to reach the editor: it still shows
  the previous document, whose cells and pages the answer would name
- a hidden tab (the same session open twice) answers a poll later, so the
  tab in front answers first; alone, it still answers within the timeout
- a cell's container is reported by the model's isLayer, not by comparing
  with the default parent, which is the group the user entered

contexts/diagram-context.tsx

@@ -115,6 +115,9 @@ interface DiagramContextType {
      * another page): not recorded as the diagram, its autosaves ignored,
      * until the next loadDiagram puts the real document back */
     showTransient: (xml: string) => void
+    /** A full load was sent and draw.io has not reported it yet: the
+     * editor still shows the previous document */
+    hasPendingLoad: () => boolean
     isDrawioReady: boolean
     onDrawioLoad: () => void
     resetDrawioReady: () => void
@@ -597,6 +600,7 @@ export function DiagramProvider({ children }: { children: React.ReactNode }) {
                 captureValidationPng,
                 requestExport,
                 showTransient,
+                hasPendingLoad: () => pendingLoadsRef.current > 0,
                 isDrawioReady,
                 onDrawioLoad,
                 resetDrawioReady,