fix(mutator): auto-fall-back to legacy builder when buildx wedges live decky add

apply_add_decky's compose-up was hard-failing whenever the operator's
~/.docker/buildx/activity/ landed on a read-only mount — the wedge
detection in _compose_with_retry correctly refuses to retry (would
just leak more mounts), but for live materialisation we don't want a
wedged buildx state to abort an admin's mutation.  ANTI hit it on
adding decky-a977: 'failed to update builder last activity time: ...
read-only file system → buildx wedge detected → returned non-zero'.

_compose_up_with_buildkit_fallback wraps _compose_with_retry: on a
CalledProcessError whose stderr matches both wedge signatures
(_BUILDX_WEDGE_SIGNATURE + _BUILDX_EROFS_SIGNATURE), it logs a
warning with the manual recovery steps + retries once with
DOCKER_BUILDKIT=0 set.  The legacy non-buildx builder doesn't use
the activity dir and isn't affected.

Wired into the two paths that pass --build:
* _materialise_decky_spawn (apply_add_decky)
* _materialise_decky_services_diff (apply_update_decky service add)

_materialise_decky_recreate_base doesn't build — it just recreates a
container from an existing image — so it's not affected.

Operator-facing log message points at the manual fix
(rm -rf ~/.docker/buildx/activity + docker buildx create) so they
can recover at their leisure; we don't ATTEMPT the recovery because
the activity dir might be RO for a reason (zfs/btrfs snapshot, etc.)
that an automated rm would be wrong to fight.

tests/mutator/test_ops_materialisation.py

@@ -298,6 +298,55 @@ async def test_update_decky_forwards_l3_flip_with_force_recreates_base(
     assert found, "expected force-recreate up against the base"
 
 
+@pytest.mark.anyio
+async def test_add_decky_falls_back_to_legacy_builder_on_buildx_wedge(
+    repo, monkeypatch, tmp_path,
+):
+    """When the first compose up hits the buildx EROFS wedge, the
+    helper retries once with DOCKER_BUILDKIT=0 (legacy builder)."""
+    import subprocess
+    from decnet.engine import deployer as _deployer
+    from decnet.topology import compose as _compose_mod
+
+    tid = await _make_active(repo)
+    lans = await repo.list_lans_for_topology(tid)
+    home_lan = lans[0]["name"]
+
+    calls: list[dict] = []
+
+    def fake_compose_with_retry(*args, compose_file=None, env=None, **_):
+        calls.append({"args": args, "env": env})
+        # First call: simulate the wedge.  Second: succeed.
+        if len(calls) == 1:
+            raise subprocess.CalledProcessError(
+                returncode=1,
+                cmd=["docker", "compose", *args],
+                output="",
+                stderr=(
+                    "failed to update builder last activity time: open "
+                    "/home/anti/.docker/buildx/activity/.tmp-X: "
+                    "read-only file system"
+                ),
+            )
+
+    monkeypatch.setattr(_deployer, "_compose_with_retry", fake_compose_with_retry)
+    monkeypatch.setattr(_deployer, "_compose", lambda *a, **k: None)
+    monkeypatch.setattr(
+        _deployer, "_topology_compose_path",
+        lambda topo_id: tmp_path / f"compose-{topo_id[:8]}.yml",
+    )
+    monkeypatch.setattr(_compose_mod, "write_topology_compose", lambda *a, **k: None)
+
+    await apply_add_decky(repo, tid, {
+        "name": "wedgey", "lan": home_lan, "services": ["ssh"],
+    })
+
+    # Two attempts total; the second carries DOCKER_BUILDKIT=0.
+    assert len(calls) == 2
+    assert calls[0]["env"] is None
+    assert calls[1]["env"] == {"DOCKER_BUILDKIT": "0"}
+
+
 @pytest.mark.anyio
 async def test_update_decky_refuses_gateway_promotion_on_non_dmz_lan(
     repo, stubs,