feat: per-tier routing activation (v0c)

RoutingPolicy gains tier_modes: a tier entry may be a string
(model, mode inherits global) or {model, mode}. The explicit
tier mode overrides the global mode in both directions, so plan
can be active while goal/default stay shadow, and one tier can
be rolled back to off without touching the others. An invalid
tier mode drops that tier entirely (fail-closed: a typo must
never accidentally activate a tier, and never kills other
tiers). Tier-off tasks return worker_default and are not
audited as policy/shadow, so they never inflate shadow sample
counts. Explicit task and worker-CLI models still win over any
tier mode.

Tests: test_routing_tier.py (5 tests).

cloud/tests/test_routing_tier.py

@@ -0,0 +1,107 @@
+"""分档激活 v0c 单测：档模式优先全局、字符串兼容、无效档模式丢档。
+
+运行（在仓库根目录下）：
+    python3 -m unittest discover -s cloud/tests -p "test_routing_tier.py" -v
+
+说明：纯函数单测，不碰库、不联网、不跑模型。关键断言：
+全局 shadow 时 plan 档 active 只切 plan 档、其余档仍影子；
+全局 active 时某档 off 只回滚该档；档模式写错时该档整档
+丢掉、不回落全局（防 typo 误激活），其余档不受影响；
+字符串档写法继续按全局模式走。
+"""
+
+import sys
+import unittest
+from pathlib import Path
+
+try:
+    from cloud.models import decide_routing, parse_routing_policy
+except ImportError:
+    sys.path.insert(0, str(Path(__file__).resolve().parents[2]))
+    from cloud.models import decide_routing, parse_routing_policy
+
+
+def _policy(data):
+    policy = parse_routing_policy(data)
+    assert policy is not None, data
+    return policy
+
+
+class TierModeTestCase(unittest.TestCase):
+    def test_plan_active_only_under_global_shadow(self):
+        policy = _policy({
+            "mode": "shadow",
+            "tiers": {
+                "plan": {"model": "gpt-6.1-sol", "mode": "active"},
+                "goal": "gpt-6.1-sol",
+                "default": "gpt-6.1-sol",
+            },
+        })
+        plan = decide_routing({"id": "t1", "plan": True}, "gpt-5.6-terra",
+                              policy=policy)
+        self.assertEqual(plan.source, "policy")
+        self.assertTrue(plan.applied)
+        self.assertEqual(plan.chosen_model, "gpt-6.1-sol")
+        goal = decide_routing({"id": "t2", "goal_mode": True},
+                              "gpt-5.6-terra", policy=policy)
+        self.assertEqual(goal.source, "shadow")
+        self.assertFalse(goal.applied)
+        self.assertEqual(goal.suggested_model, "gpt-6.1-sol")
+        self.assertEqual(goal.chosen_model, "gpt-5.6-terra")
+
+    def test_tier_off_rolls_back_only_that_tier(self):
+        policy = _policy({
+            "mode": "active",
+            "tiers": {
+                "plan": {"model": "gpt-6.1-sol", "mode": "off"},
+                "default": "gpt-6.1-sol",
+            },
+        })
+        plan = decide_routing({"id": "t1", "plan": True}, "gpt-5.6-terra",
+                              policy=policy)
+        self.assertEqual(plan.source, "worker_default")
+        self.assertEqual(plan.chosen_model, "gpt-5.6-terra")
+        default = decide_routing({"id": "t2"}, "gpt-5.6-terra",
+                                 policy=policy)
+        self.assertEqual(default.source, "policy")
+        self.assertEqual(default.chosen_model, "gpt-6.1-sol")
+
+    def test_string_tier_inherits_global_mode(self):
+        policy = _policy({
+            "mode": "shadow",
+            "tiers": {"plan": "gpt-6.1-sol"},
+        })
+        decision = decide_routing({"id": "t1", "plan": True},
+                                  "gpt-5.6-terra", policy=policy)
+        self.assertEqual(decision.source, "shadow")
+        self.assertFalse(decision.applied)
+
+    def test_invalid_tier_mode_drops_tier_not_global(self):
+        policy = _policy({
+            "mode": "active",
+            "tiers": {
+                "plan": {"model": "gpt-6.1-sol", "mode": "activ"},
+                "default": "gpt-6.1-sol",
+            },
+        })
+        self.assertNotIn("plan", policy.tiers)
+        plan = decide_routing({"id": "t1", "plan": True}, "gpt-5.6-terra",
+                              policy=policy)
+        # 丢档 = 回 worker 默认，不按全局 active 误激活。
+        self.assertEqual(plan.source, "worker_default")
+        self.assertEqual(plan.chosen_model, "gpt-5.6-terra")
+        default = decide_routing({"id": "t2"}, "gpt-5.6-terra",
+                                 policy=policy)
+        self.assertEqual(default.source, "policy")
+
+    def test_explicit_task_and_worker_still_win(self):
+        policy = _policy({
+            "mode": "active",
+            "tiers": {"plan": {"model": "gpt-6.1-sol",
+                               "mode": "active"}},
+        })
+        explicit = decide_routing(
+            {"id": "t1", "plan": True, "model": "gpt-5.6-terra"},
+            "gpt-5.6-terra", policy=policy)
+        self.assertEqual(explicit.source, "explicit")
+        self.assertEqual(explicit.chosen_model, "gpt-5.6-terra")