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

lib/drawio/editor-bridge.ts

@@ -610,7 +610,6 @@ 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) => {
@@ -635,8 +634,11 @@ export function readSelectionDetails(): SelectionAnswer | null {
                     }
                 }
             }
+            // The container the cell is in; a layer is none (and the
+            // default parent is the group the user entered, if any, so it
+            // cannot tell the two apart)
             const parent = g.model.getParent(cell)
-            if (parent?.id && parent !== defaultParent) {
+            if (parent?.id && !g.model.isLayer(parent)) {
                 info.parent = String(parent.id)
             }
             return info