feat: pack recent submissions into reserved account slots first

Preserve shared queue positions for old-model candidates with constrained
admission. Retain headroom balancing for ties and weekly calibration pinning.
Add offline mixed-age allocation regressions.

tests/test_concurrency.py

@@ -217,6 +217,43 @@ def make_candidate(index: int) -> dict:
 
 
 class ClientPoolConcurrencyTests(unittest.TestCase):
+    def test_recent_work_preserves_shared_slots_for_later_old_models(self) -> None:
+        for initial_counts, ages, expected_counts in (
+            ([94, 95], ["recent", "old"], [95, 96]),
+            ([93, 94], ["recent", "recent", "old", "old"], [95, 96]),
+        ):
+            with self.subTest(initial_counts=initial_counts):
+                clients = [FakeClient(active_count=count) for count in initial_counts]
+                pool = ModelHubClientPool(
+                    clients,  # type: ignore[arg-type]
+                    active_task_cap=100,
+                    recent_model_reserve_slots=5,
+                    active_counts_ttl=60,
+                    capacity_probe_interval_cycles=0,
+                )
+                for index, age in enumerate(ages):
+                    pool.add_task_for_model(
+                        {"model": f"model-{index}"},
+                        model_last_modified=(
+                            "2026-10-01T00:00:00Z" if age == "recent"
+                            else "2025-01-01T00:00:00Z"
+                        ),
+                        submitted_at="2026-10-02T00:00:00Z",
+                    )
+                self.assertEqual(expected_counts, pool.active_task_counts())
+                self.assertEqual(len(ages), sum(len(client.submitted) for client in clients))
+                self.assertEqual(0, pool.old_model_submit_slots())
+
+    def test_recent_packing_keeps_largest_headroom_without_reserved_slots(self) -> None:
+        clients = [FakeClient(active_count=2), FakeClient(active_count=3)]
+        pool = ModelHubClientPool(
+            clients,  # type: ignore[arg-type]
+            active_task_cap=5,
+            recent_model_reserve_slots=0,
+        )
+        pool.add_task({"model": "recent"})
+        self.assertEqual([1, 0], [len(client.submitted) for client in clients])
+
     def test_stale_capacity_rejection_does_not_poison_learned_limit(self) -> None:
         with tempfile.TemporaryDirectory() as temporary_dir:
             state_path = Path(temporary_dir) / "capacity.json"