marker: concurrent (bounded) definition fan-out

The per-definition fan-out applied markers to links in a serial loop -- one compute round-trip per link. On a 1000-link project that serializes N HTTP round-trips (minutes on remote computes). Fan out with asyncio.gather + Semaphore(32): links are independent (own _markers/_link_data), per-link ControllerError stays isolated, and Project.dump is synchronous + atomic (tmp + rename) so concurrent dumps cannot corrupt the topology file.

Converts the definition-create fan-out, the definition-update sync and re-fan-out loops, and the definition-delete cleanup to the shared _marker_apply_concurrently helper. apply_defs_to_new_link stays serial deliberately: all definitions share one link and each push carries the link's full marker set, so concurrent pushes would race and lose markers.

tests/controller/test_marker.py

@@ -433,6 +433,21 @@ async def test_create_marker_definition_fans_out(project):
     assert project.marker_definitions["arp"]["highlight_duration"] == 1200
 
 
+@pytest.mark.asyncio
+async def test_definition_fans_out_over_many_links(project):
+    # The fan-out is concurrent (bounded) — a large topology must not serialize
+    # N compute round-trips — but behaviorally every link still receives the
+    # marker and per-link failures stay isolated.
+    with _valid_bpf():
+        links = [await _make_link(project) for _ in range(20)]
+        await project.create_marker_definition("arp", "arp", highlight_duration=700)
+
+    for link in links:
+        assert link.markers["global-arp"]["highlight_duration"] == 700
+        assert link.markers["global-arp"]["inherited_from"] == "arp"
+    assert project.marker_definitions["arp"]["highlight_duration"] == 700
+
+
 @pytest.mark.asyncio
 async def test_update_marker_definition_syncs(project):