refactor(topology_ops): decompose apply() into focused helpers

apply() was an 85-line function handling hash verification, validation,
superseding teardown, bridge/compose provisioning, and store persistence.
Extracted _check_hash_and_validate(), _teardown_superseded(), and _materialise()
so each step is independently readable and testable.

decnet/agent/topology_ops.py

@@ -59,6 +59,73 @@ def _topology_id(hydrated: dict[str, Any]) -> str:
     return str(tid)
 
 
+def _check_hash_and_validate(hydrated: dict[str, Any], version_hash: str) -> str:
+    """Verify hash integrity and structural validity; return topology_id."""
+    local_hash = canonical_hash(hydrated)
+    if local_hash != version_hash:
+        raise HashMismatch(
+            f"master hash {version_hash!r} does not match agent hash "
+            f"{local_hash!r} — refusing to apply"
+        )
+    issues = _validate_topology(hydrated)
+    if _validation_errors(issues):
+        raise ValidationError(issues)
+    return _topology_id(hydrated)
+
+
+async def _teardown_superseded(topology_id: str, store: TopologyStore) -> None:
+    """Tear down the current topology if it differs from topology_id.
+
+    Master is authoritative — a different pinned topology (fully applied,
+    partially applied, or drifted) is torn down before the new apply proceeds.
+    Refusing with 409 would leave the agent stuck in a state only a human
+    could resolve.
+    """
+    existing = store.current()
+    if existing is None or existing.topology_id == topology_id:
+        return
+    log.info(
+        "superseding topology %s with %s on master authority",
+        existing.topology_id, topology_id,
+    )
+    try:
+        await teardown(existing.topology_id, store)
+    except Exception as exc:  # noqa: BLE001 — we still want to try applying
+        log.warning(
+            "best-effort teardown of superseded topology %s failed: %s",
+            existing.topology_id, exc,
+        )
+        # Hard-clear the store row so the new apply isn't blocked by a
+        # half-torn-down predecessor.  Leftover docker objects surface via
+        # the next heartbeat's observed block.
+        store.clear(existing.topology_id)
+
+
+def _materialise(hydrated: dict[str, Any], topology_id: str) -> None:
+    """Create bridge networks, write compose file, and bring up containers.
+
+    Sync/blocking — callers must dispatch via asyncio.to_thread.
+
+    ``--always-recreate-deps`` keeps service containers' netns shares
+    fresh: every decky service joins its base's netns via
+    ``network_mode: container:<base>``, and that share is bound at
+    service start time. If a base is recreated (e.g. when ``ports:``
+    changes after toggling ``forwards_l3``) but compose decides the
+    services are unchanged, the services keep a stale netns FD
+    pointing at the destroyed base — they end up in an empty
+    namespace with only ``lo``, and external traffic hits a closed
+    port on the live base. Forcing dependents to recreate alongside
+    the base is the cheapest way to make this race impossible.
+    """
+    compose_path = _topology_compose_path(topology_id)
+    client = docker.from_env()
+    for lan in hydrated["lans"]:
+        net_name = _topology_network_name(topology_id, lan["name"])
+        create_bridge_network(client, net_name, lan["subnet"], internal=not lan["is_dmz"])
+    write_topology_compose(hydrated, compose_path)
+    _compose_with_retry("up", "--build", "-d", "--always-recreate-deps", compose_file=compose_path)
+
+
 async def apply(
     hydrated: dict[str, Any],
     version_hash: str,
@@ -73,76 +140,11 @@ async def apply(
       Any docker / compose error propagates up; the endpoint maps it
         to 500 and records the message on the store row.
     """
-    local_hash = canonical_hash(hydrated)
-    if local_hash != version_hash:
-        raise HashMismatch(
-            f"master hash {version_hash!r} does not match agent hash "
-            f"{local_hash!r} — refusing to apply"
-        )
-
-    issues = _validate_topology(hydrated)
-    if _validation_errors(issues):
-        raise ValidationError(issues)
-
-    topology_id = _topology_id(hydrated)
-    # Master is authoritative.  If a different topology is pinned here
-    # — whether it fully applied, only partially applied (failure
-    # marker row + orphan containers), or drifted — teardown first,
-    # then accept the new one.  Refusing with 409 would leave the
-    # agent stuck in a state only a human could resolve.
-    existing = store.current()
-    if existing is not None and existing.topology_id != topology_id:
-        log.info(
-            "superseding topology %s with %s on master authority",
-            existing.topology_id, topology_id,
-        )
-        try:
-            await teardown(existing.topology_id, store)
-        except Exception as exc:  # noqa: BLE001 — we still want to try applying
-            log.warning(
-                "best-effort teardown of superseded topology %s failed: %s",
-                existing.topology_id, exc,
-            )
-            # Hard-clear the store row so the new apply isn't blocked
-            # by a half-torn-down predecessor.  Leftover docker objects
-            # will surface via the next heartbeat's observed block.
-            store.clear(existing.topology_id)
-
-    lans = hydrated["lans"]
-    compose_path = _topology_compose_path(topology_id)
-    client = docker.from_env()
-
-    # Bridges + compose are sync/blocking; hop to a thread so we don't
-    # stall the event loop on a slow docker daemon.
-    def _materialise() -> None:
-        for lan in lans:
-            net_name = _topology_network_name(topology_id, lan["name"])
-            internal = not lan["is_dmz"]
-            create_bridge_network(
-                client, net_name, lan["subnet"], internal=internal
-            )
-        write_topology_compose(hydrated, compose_path)
-        # ``--always-recreate-deps`` keeps service containers' netns shares
-        # fresh: every decky service joins its base's netns via
-        # ``network_mode: container:<base>``, and that share is bound at
-        # service start time. If a base is recreated (e.g. when ``ports:``
-        # changes after toggling ``forwards_l3``) but compose decides the
-        # services are unchanged, the services keep a stale netns FD
-        # pointing at the destroyed base — they end up in an empty
-        # namespace with only ``lo``, and external traffic hits a closed
-        # port on the live base. Forcing dependents to recreate alongside
-        # the base is the cheapest way to make this race impossible.
-        _compose_with_retry(
-            "up", "--build", "-d", "--always-recreate-deps",
-            compose_file=compose_path,
-        )
-
-    await asyncio.to_thread(_materialise)
-
+    topology_id = _check_hash_and_validate(hydrated, version_hash)
+    await _teardown_superseded(topology_id, store)
+    await asyncio.to_thread(_materialise, hydrated, topology_id)
     store.put(topology_id, version_hash, hydrated)
-    log.info(
-        "topology %s applied on agent (%d LANs)", topology_id, len(lans)
-    )
+    log.info("topology %s applied on agent (%d LANs)", topology_id, len(hydrated["lans"]))
 
 
 async def teardown(