feat(enroll): opt-in IPvlan per-agent for Wi-Fi-bridged VMs

Wi-Fi APs bind one MAC per associated station, so VirtualBox/VMware
guests bridged over Wi-Fi rotate the VM's DHCP lease when Docker's
macvlan starts emitting container-MAC frames through the vNIC. Adds a
`use_ipvlan` toggle on the Agent Enrollment tab (mirrors the updater
daemon checkbox): flips the flag on SwarmHost, bakes `ipvlan=true` into
the agent's decnet.ini, and `_worker_config` forces ipvlan=True on the
per-host shard at dispatch. Safe no-op on wired/bare-metal agents.

decnet/web/router/swarm_mgmt/api_enroll_bundle.py

@@ -87,6 +87,15 @@ class EnrollBundleRequest(BaseModel):
         default=True,
         description="Include updater cert bundle and auto-start decnet updater on the agent",
     )
+    use_ipvlan: bool = Field(
+        default=False,
+        description=(
+            "Run deckies on this agent over IPvlan L2 instead of MACVLAN. "
+            "Required when the agent is a VirtualBox/VMware guest bridged over Wi-Fi — "
+            "Wi-Fi APs bind one MAC per station, so MACVLAN's extra container MACs "
+            "rotate the VM's DHCP lease. Safe no-op on wired/bare-metal hosts."
+        ),
+    )
     services_ini: Optional[str] = Field(
         default=None,
         description="Optional INI text shipped to the agent as /etc/decnet/services.ini",
@@ -166,13 +175,15 @@ def _is_excluded(rel: str) -> bool:
     return False
 
 
-def _render_decnet_ini(master_host: str) -> bytes:
+def _render_decnet_ini(master_host: str, use_ipvlan: bool = False) -> bytes:
+    ipvlan_line = f"ipvlan = {'true' if use_ipvlan else 'false'}\n"
     return (
         "; Generated by DECNET agent-enrollment bundle.\n"
         "[decnet]\n"
         "mode = agent\n"
         "disallow-master = true\n"
         "log-directory = /var/log/decnet\n"
+        f"{ipvlan_line}"
         "\n"
         "[agent]\n"
         f"master-host = {master_host}\n"
@@ -197,6 +208,7 @@ def _build_tarball(
     issued: pki.IssuedCert,
     services_ini: Optional[str],
     updater_issued: Optional[pki.IssuedCert] = None,
+    use_ipvlan: bool = False,
 ) -> bytes:
     """Gzipped tarball with:
       - full repo source (minus excludes)
@@ -216,7 +228,7 @@ def _build_tarball(
                 continue
             tar.add(path, arcname=rel, recursive=False)
 
-        _add_bytes(tar, "etc/decnet/decnet.ini", _render_decnet_ini(master_host))
+        _add_bytes(tar, "etc/decnet/decnet.ini", _render_decnet_ini(master_host, use_ipvlan))
         for unit in _SYSTEMD_UNITS:
             _add_bytes(
                 tar,
@@ -329,11 +341,15 @@ async def create_enroll_bundle(
             "cert_bundle_path": str(bundle_dir),
             "enrolled_at": datetime.now(timezone.utc),
             "notes": "enrolled via UI bundle",
+            "use_ipvlan": req.use_ipvlan,
         }
     )
 
     # 3. Render payload + bootstrap.
-    tarball = _build_tarball(req.master_host, req.agent_name, issued, req.services_ini, updater_issued)
+    tarball = _build_tarball(
+        req.master_host, req.agent_name, issued, req.services_ini, updater_issued,
+        use_ipvlan=req.use_ipvlan,
+    )
     token = secrets.token_urlsafe(24)
     expires_at = datetime.now(timezone.utc) + BUNDLE_TTL