observer: wire into forwarding/output paths for live capture

Router now captures packets flowing through the network stack:
- forward_packets(): capture all forwarded/local-delivered packets
- Observer injected via Router::new() from Smolnetd constructor

When /scheme/netcfg/capture/enable is written, all packets
traversing the router are captured into the ring buffer.
When disabled, zero overhead (AtomicBool check).

netstack/src/router/mod.rs

@@ -10,6 +10,7 @@ use self::route_table::{RouteTable, RouteType};
 use crate::filter::{FilterTable, Hook, PacketContext, Verdict};
 use crate::icmp_error;
 use crate::link::DeviceList;
+use crate::observer::ObserverRef;
 use crate::scheme::Smolnetd;
 
 pub mod route_table;
@@ -22,10 +23,11 @@ pub struct Router {
     devices: Rc<RefCell<DeviceList>>,
     route_table: Rc<RefCell<RouteTable>>,
     pub ip_forward: Rc<Cell<bool>>,
+    observer: ObserverRef,
 }
 
 impl Router {
-    pub fn new(devices: Rc<RefCell<DeviceList>>, route_table: Rc<RefCell<RouteTable>>) -> Self {
+    pub fn new(devices: Rc<RefCell<DeviceList>>, route_table: Rc<RefCell<RouteTable>>, observer: ObserverRef) -> Self {
         let rx_buffer = PacketBuffer::new(
             vec![PacketMetadata::EMPTY; Smolnetd::SOCKET_BUFFER_SIZE],
             vec![0u8; Router::MTU * Smolnetd::SOCKET_BUFFER_SIZE],
@@ -40,6 +42,7 @@ impl Router {
             devices,
             route_table,
             ip_forward: Rc::new(Cell::new(true)),
+            observer,
         }
     }
 
@@ -185,6 +188,13 @@ impl Router {
             forwarded.push(buf);
         }
 
+        for packet in &forwarded {
+            self.observer.capture(packet);
+        }
+        for packet in &local {
+            self.observer.capture(packet);
+        }
+
         for packet in local {
             let Ok(buf) = self.rx_buffer.enqueue(packet.len(), ()) else {
                 break;

netstack/src/scheme/mod.rs

@@ -113,13 +113,13 @@ impl Smolnetd {
 
         let devices = Rc::new(RefCell::new(DeviceList::default()));
         let route_table = Rc::new(RefCell::new(RouteTable::default()));
+        let observer = Observer::new();
         let mut network_device = Tracer::new(
-            Router::new(Rc::clone(&devices), Rc::clone(&route_table)),
+            Router::new(Rc::clone(&devices), Rc::clone(&route_table), Rc::clone(&observer)),
             |_timestamp, printer| trace!("{}", printer),
         );
 
         let ip_forward = network_device.get_mut().ip_forward.clone();
-        let observer = Observer::new();
 
         let config = Config::new(HardwareAddress::Ip);
         let mut iface = SmoltcpInterface::new(config, &mut network_device, Instant::now());