fix(controller): re-arm window_drop and expose kernel_datapath

Three fixes from live-calibrating the kernel impairment filters on an
isolated instance (Docker-to-Docker veth link, per-sequence evidence):

- update_filters skipped an unchanged filters dict as a no-op, so a
  one-shot window_drop outage that had already passed could never be
  re-armed: re-applying the same values produced no datapath command at
  all. A PUT still carrying window_drop now reconciles, restarting the
  schedule from the apply as documented. Measured: identical re-apply
  moves the outage window from seq 50-70 to 70-91 (arm at +8s of the
  new apply).

- gemodel parameter semantics were documented backwards in the
  validation rules, available_filters, the MCP/copilot descriptions and
  the feature docs. Measured against the kernel (per-direction counters,
  offered = sent + dropped): p is the good-to-bad transition chance, r
  the bad-to-good transition, 1-h the loss chance in the bad state (the
  good state loses nothing) - mean one-way loss p/(p+r)*(1-h), e.g.
  [10,30,100] -> 24.8%/25.7% measured vs 25% predicted, [5,45,100] ->
  10.8% both ways vs 10%.

- kernel_datapath was silently stripped from every REST/MCP link payload
  by the response model (LinkBase carried no such field), so clients
  could not tell a kernel link from a relay one.

Also verified on the same setup: the 3-argument window_drop form works
exactly as specified ([3000,5000,100] -> a single seq 31-55 outage,
[20000,6000,100] -> seq 115-144), and both generations of the command
builder emit the parameters unpadded.

Adds an API test for the field and controller tests for the no-op skip
and the window_drop re-arm exemption.

tests/controller/test_link.py

@@ -361,6 +361,58 @@ async def test_update_filters(project, compute):
     assert link.update.called
 
 
+@pytest.mark.asyncio
+async def test_update_filters_unchanged_is_noop(project, compute):
+    """Re-sending an identical filters dict skips the datapath reconcile."""
+
+    node1 = Node(project, compute, "node1", node_type="qemu")
+    node1._ports = [EthernetPort("E0", 0, 0, 4)]
+    node2 = Node(project, compute, "node2", node_type="qemu")
+    node2._ports = [EthernetPort("E0", 0, 0, 4)]
+
+    link = Link(project)
+    link.create = AsyncioMagicMock()
+    link._project.emit_notification = MagicMock()
+    project.dump = AsyncioMagicMock()
+    await link.add_node(node1, 0, 4)
+    await link.add_node(node2, 0, 4)
+
+    link.update = AsyncioMagicMock()
+    await link.update_filters({"packet_loss": [10]})
+    assert link.update.call_count == 1
+    await link.update_filters({"packet_loss": [10]})
+    assert link.update.call_count == 1
+
+
+@pytest.mark.asyncio
+async def test_update_filters_unchanged_window_drop_still_reconciles(project, compute):
+    """
+    window_drop's outage schedule is measured from the apply, so an
+    unchanged re-send must re-arm it instead of being skipped as a no-op.
+    """
+
+    node1 = Node(project, compute, "node1", node_type="qemu")
+    node1._ports = [EthernetPort("E0", 0, 0, 4)]
+    node2 = Node(project, compute, "node2", node_type="qemu")
+    node2._ports = [EthernetPort("E0", 0, 0, 4)]
+
+    link = Link(project)
+    link.create = AsyncioMagicMock()
+    link._project.emit_notification = MagicMock()
+    project.dump = AsyncioMagicMock()
+    await link.add_node(node1, 0, 4)
+    await link.add_node(node2, 0, 4)
+    # window_drop is kernel-only: this synthetic link has no real wiring,
+    # so declare the kernel datapath the way _link_data does at runtime
+    link._link_data = [{"type": "nio_bridge"}]
+
+    link.update = AsyncioMagicMock()
+    await link.update_filters({"window_drop": [3000, 5000, 100]})
+    assert link.update.call_count == 1
+    await link.update_filters({"window_drop": [3000, 5000, 100]})
+    assert link.update.call_count == 2
+
+
 @pytest.mark.asyncio
 async def test_available_filters(project, compute):