STATE-WP-0069 T04: meter workstream_id query-param aliases

Add legacy_compat helper and Deprecation headers when callers filter
/tasks, /decisions, /token-events, or /execution/launch-requests with
workstream_id. Preferred workplan_id filters are unchanged. Route removal
remains gated on legacy-meter zero-usage windows.

tests/test_legacy_meter.py

@@ -143,6 +143,46 @@ class TestWorkplanAliasesAndLegacyMeter:
         assert event["interface"]["replacement_ref"] == "org.statehub.workplan.completed"
         assert event["window"]["components"] == {"state-hub.events": 1}
 
+    async def test_legacy_workstream_id_query_param_on_tasks_is_metered(self, client):
+        await _create_domain(client)
+        topic = await _create_topic(client)
+        wp = await _create_workplan(client, topic["id"])
+        task_r = await client.post("/tasks/", json={
+            "workplan_id": wp["id"],
+            "title": "Meter me",
+            "status": "todo",
+        })
+        assert task_r.status_code == 201, task_r.text
+
+        r = await client.get(
+            f"/tasks/?workstream_id={wp['id']}",
+            headers={
+                "X-StateHub-Tenant": "tenant-a",
+                "X-StateHub-User": "alice",
+                "X-StateHub-Component": "old-task-client",
+            },
+        )
+        assert r.status_code == 200
+        assert r.headers["Deprecation"] == "true"
+        assert r.headers["X-StateHub-Replacement"] == "/tasks/?workplan_id=<workplan_id>"
+
+        summary = (await client.get("/legacy-meter/summary")).json()
+        item = _summary_by_key(summary)["rest_api:GET /tasks/?workstream_id"]
+        assert item["window"]["calls"] == 1
+        assert item["window"]["components"] == {"old-task-client": 1}
+
+    async def test_workplan_id_query_param_on_tasks_is_not_metered(self, client):
+        await _create_domain(client)
+        topic = await _create_topic(client)
+        wp = await _create_workplan(client, topic["id"])
+
+        r = await client.get(f"/tasks/?workplan_id={wp['id']}")
+        assert r.status_code == 200
+        assert r.headers.get("Deprecation") != "true"
+
+        summary = (await client.get("/legacy-meter/summary")).json()
+        assert _summary_by_key(summary).get("rest_api:GET /tasks/?workstream_id") is None
+
     async def test_workplan_dependency_and_execution_aliases(self, client):
         await _create_domain(client)
         topic = await _create_topic(client)