nat: active SNAT binding tracking + display

NatTable gains bindings: Vec<NatBinding> field. record_snat()
called after successful SNAT rewrite captures original→translated
src_addr/port mapping. Bindings capped at 1024 entries (FIFO).

format_bindings() returns:
  Active SNAT bindings: 3
    10.0.0.1:1234 -> 192.168.1.100:40001
    10.0.0.2:5678 -> 192.168.1.100:40002

Useful for diagnosing NAT issues and seeing which internal
connections are being source-NAT'd. Mirrors Linux
/proc/net/ip_conntrack display.

All 31 tests pass.

netstack/src/filter/nat.rs

@@ -70,6 +70,7 @@ impl Default for NatBinding {
 #[derive(Debug)]
 pub struct NatTable {
     pub rules: Vec<NatRule>,
+    pub bindings: Vec<NatBinding>,
     next_id: u32,
     ephemeral_port: u16,
 }
@@ -78,6 +79,7 @@ impl NatTable {
     pub fn new() -> Self {
         Self {
             rules: Vec::new(),
+            bindings: Vec::new(),
             next_id: 1,
             ephemeral_port: 40000,
         }
@@ -117,6 +119,31 @@ impl NatTable {
         None
     }
 
+    /// Record an active SNAT binding for display purposes.
+    /// Called after a successful SNAT rewrite.
+    pub fn record_snat(
+        &mut self,
+        orig_src: IpAddress,
+        trans_src: IpAddress,
+        orig_sport: u16,
+        trans_sport: u16,
+    ) {
+        // Cap bindings to prevent unbounded growth.
+        if self.bindings.len() >= 1024 {
+            self.bindings.remove(0);
+        }
+        self.bindings.push(NatBinding {
+            orig_src,
+            trans_src,
+            orig_dst: IpAddress::v4(0, 0, 0, 0),
+            trans_dst: IpAddress::v4(0, 0, 0, 0),
+            orig_sport,
+            trans_sport,
+            orig_dport: 0,
+            trans_dport: 0,
+        });
+    }
+
     pub fn lookup_dnat(
         &self,
         hook: Hook,
@@ -162,6 +189,20 @@ impl NatTable {
         out
     }
 
+    pub fn format_bindings(&self) -> String {
+        if self.bindings.is_empty() {
+            return "no active bindings\n".to_string();
+        }
+        let mut out = format!("Active SNAT bindings: {}\n", self.bindings.len());
+        for b in &self.bindings {
+            out.push_str(&alloc::format!(
+                "  {}:{} -> {}:{}\n",
+                b.orig_src, b.orig_sport, b.trans_src, b.trans_sport,
+            ));
+        }
+        out
+    }
+
     pub fn remove(&mut self, id: u32) -> bool {
         if let Some(idx) = self.rules.iter().position(|r| r.id == id) {
             self.rules.remove(idx);