feat(news): index news packages and research persistent events

Extract reusable literal keywords once per source revision, assign complete news packages to long-lived events, and research all event members with immutable evidence and references.

Add additive research storage, vocabulary snapshots, the two-stage workspace, isolated acceptance coverage, and a paused rollout with an initial fixed 50-news production trial.

backend/tests/test_news_sample_replay.py

@@ -0,0 +1,46 @@
+"""Replay orchestration must drain or cancel every synthetic task."""
+import asyncio
+import importlib.util
+from pathlib import Path
+import pytest
+
+
+def load_replay():
+    path=Path(__file__).resolve().parents[2]/'scripts/replay_news_sample.py'
+    spec=importlib.util.spec_from_file_location('sample_replay_test',path)
+    module=importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+    return module
+
+
+def test_replay_refills_before_slow_task_finishes(trading):
+    replay=load_replay()
+    async def run():
+        third_started=asyncio.Event()
+        order=[]
+        async def action(identifier):
+            if identifier == 1: await third_started.wait()
+            if identifier == 3: third_started.set()
+            order.append(identifier)
+        await asyncio.wait_for(replay.bounded([1,2,3],action,2),1)
+        assert order.index(3)<order.index(1)
+    asyncio.run(run())
+
+
+def test_replay_failure_cancels_and_drains_other_tasks(trading):
+    replay=load_replay()
+    async def run():
+        started=asyncio.Event()
+        drained=[]
+        async def action(identifier):
+            if identifier == 1:
+                await started.wait()
+                raise ValueError('synthetic failure')
+            try:
+                started.set()
+                await asyncio.Event().wait()
+            finally: drained.append(identifier)
+        with pytest.raises(ValueError,match='synthetic failure'):
+            await replay.bounded([1,2],action,2)
+        assert drained==[2]
+    asyncio.run(run())