feat(consistency): rebuild deterministic projection IDs

Assistant: codex
Assistant-Model: gpt-5.6-sol
Assistant-Session: 01a023c0-a0a3-7c03-b395-5a0d2757214d

tests/test_routers_core.py

@@ -7,6 +7,8 @@ the FastAPI ASGI app.
 """
 from __future__ import annotations
 
+import uuid
+
 import pytest
 
 
@@ -139,6 +141,30 @@ class TestTopics:
 # ---------------------------------------------------------------------------
 
 class TestWorkstreams:
+    async def test_create_preserves_authoritative_projection_ids(self, client):
+        await _create_domain(client)
+        repo = await _create_repo(client, slug="projection-id-repo")
+        workplan_id = str(uuid.uuid4())
+        workplan = await _create_workplan(
+            client,
+            repo["id"],
+            slug="projection-id-wp",
+            id=workplan_id,
+        )
+        assert workplan["id"] == workplan_id
+
+        task_id = str(uuid.uuid4())
+        response = await client.post(
+            "/tasks/",
+            json={
+                "id": task_id,
+                "workplan_id": workplan_id,
+                "title": "File-backed task",
+            },
+        )
+        assert response.status_code == 201, response.text
+        assert response.json()["id"] == task_id
+
     async def test_create_and_list_by_topic(self, client):
         await _create_domain(client)
         topic = await _create_topic(client)