route: flush node — clear entire routing table

New /scheme/netcfg/route/flush clears all routes at once:
  echo > /scheme/netcfg/route/flush

Mirrors Linux 'ip route flush all'.

Replaces the route table with a new empty RouteTable. All
existing Rc shares see the change immediately. Useful for
bulk route management (clear and re-add). Notifies route/list
subscribers after flush.

All 31 tests pass.

netstack/src/scheme/netcfg/mod.rs

@@ -128,7 +128,7 @@ fn mk_root_node(
                  nat        — NAT bindings (bindings / stats)\n\
                  capture    — packet capture (enable/read/filter/count)\n\
                  sysctl     — kernel tunables (net/ipv4/ip_forward)\n\
-                 route      — routing table (list/add/rm/count/gateway)\n\
+                 route      — routing table (list/add/rm/flush/count/gateway)\n\
                  resolv     — DNS configuration (nameserver/nameserver6)\n\
                  ifaces     — per-interface config/stats (eth0/lo/...)\n\
                  version    — netstack version info\n"
@@ -566,9 +566,20 @@ fn mk_root_node(
                                 notifier.borrow_mut().schedule_notify("route/list");
                             }
                             Ok(())
-                        }
-                    },
-                },
+                }
+            },
+            "flush" => {
+                wo [route_table, notifier] (Option<()>, None)
+                |_cur_value, _line| {
+                    Ok(())
+                }
+                |_cur_value| {
+                    *route_table.borrow_mut() = RouteTable::default();
+                    notifier.borrow_mut().schedule_notify("route/list");
+                    Ok(())
+                }
+            },
+        },
                 "arp" => {
                     "list" => {
                         ro [devices] || {