feat(manifest): add allow-listed per-netns sysctls primitive

Routing apps (a rootless VPN exit) need packet forwarding in their own
network namespace, but /proc/sys is read-only inside a rootless
container, so it can only be set at create time. Add `app.sysctls`,
allow-listed to net.ipv4.ip_forward / net.ipv6.conf.all.forwarding with
values "0"/"1", and rejected under host networking where it would change
the host. Rendered on all three create paths: podman CLI --sysctl, the
libpod spec `sysctl` map, and Quadlet `Sysctl=`. Absent by default and
not serialized when empty, so existing manifests and units are unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

core/archipelago/src/container/quadlet.rs

@@ -176,6 +176,8 @@ pub struct QuadletUnit {
     /// for rotation-drift detection.
     pub labels: Vec<(String, String)>,
     pub devices: Vec<String>,
+    /// Namespaced sysctls (`Sysctl=k=v`), already allow-listed by the manifest.
+    pub sysctls: Vec<(String, String)>,
     pub add_hosts: Vec<(String, String)>,
     pub network_aliases: Vec<String>,
     pub entrypoint: Option<Vec<String>>,
@@ -307,6 +309,9 @@ impl QuadletUnit {
         for dev in &self.devices {
             let _ = writeln!(s, "AddDevice={dev}");
         }
+        for (k, v) in &self.sysctls {
+            let _ = writeln!(s, "Sysctl={k}={v}");
+        }
         for (name, ip) in &self.add_hosts {
             let _ = writeln!(s, "AddHost={name}:{ip}");
         }
@@ -521,6 +526,11 @@ impl QuadletUnit {
                 })
                 .collect(),
             devices: app.devices.clone(),
+            sysctls: app
+                .sysctls
+                .iter()
+                .map(|(k, v)| (k.clone(), v.clone()))
+                .collect(),
             add_hosts: vec![("host.archipelago".into(), "10.89.0.1".into())],
             // Container always answers to its own name; manifest extras add the
             // short hostnames peers bake in (e.g. indeedhub api/minio/relay).
@@ -1487,6 +1497,7 @@ app:
                 "RELAY_NAME=Archipelago Nostr Relay".into(),
             ],
             devices: vec!["/dev/kvm".into()],
+            sysctls: vec![("net.ipv4.ip_forward".into(), "1".into())],
             add_hosts: vec![("host.archipelago".into(), "10.89.0.1".into())],
             entrypoint: Some(vec!["/usr/local/bin/bitcoind".into()]),
             command: vec!["-server=1".into(), "-rpcbind=0.0.0.0".into()],
@@ -1503,6 +1514,7 @@ app:
         assert!(s.contains("Environment=BITCOIN_RPC_PASS=secret"));
         assert!(s.contains("Environment=\"RELAY_NAME=Archipelago Nostr Relay\""));
         assert!(s.contains("AddDevice=/dev/kvm"));
+        assert!(s.contains("Sysctl=net.ipv4.ip_forward=1"));
         assert!(s.contains("AddHost=host.archipelago:10.89.0.1"));
         assert!(s.contains("ReadOnly=true"));
         assert!(s.contains("NoNewPrivileges=true"));
@@ -1524,6 +1536,7 @@ app:
         assert!(!s.contains("PublishPort="));
         assert!(!s.contains("Environment="));
         assert!(!s.contains("AddDevice="));
+        assert!(!s.contains("Sysctl="));
         assert!(!s.contains("AddHost="));
         assert!(!s.contains("ReadOnly="));
         assert!(!s.contains("NoNewPrivileges="));
@@ -1621,6 +1634,31 @@ app:
         assert!(!s.contains("Network=host"));
     }
 
+    #[test]
+    fn from_manifest_renders_namespaced_sysctls() {
+        let yaml = r#"
+app:
+  id: vpn-exit
+  name: VPN Exit
+  version: 1.0.0
+  container:
+    image: test/vpn:1.0.0
+    network: pasta
+  devices: [/dev/net/tun]
+  sysctls:
+    net.ipv4.ip_forward: "1"
+  security:
+    capabilities: [NET_ADMIN, NET_RAW]
+"#;
+        let m = AppManifest::parse(yaml).expect("manifest must parse");
+        let s = QuadletUnit::from_manifest(&m, "vpn-exit").render();
+
+        assert!(s.contains("Network=pasta"));
+        assert!(s.contains("AddDevice=/dev/net/tun"));
+        assert!(s.contains("Sysctl=net.ipv4.ip_forward=1"));
+        assert!(s.contains("AddCapability=NET_ADMIN"));
+    }
+
     #[test]
     fn portainer_catalog_network_repairs_same_node_routing_without_exposing_backend() {
         let manifest = AppManifest::parse(include_str!("../../../../apps/portainer/manifest.yml"))