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.

modelhub_submmit_api/modelhub_client.py

@@ -948,6 +948,26 @@ class ModelHubClientPool:
             usable = {index: remaining for index, remaining in remaining_by_index.items() if remaining > 0}
             if not usable:
                 return None
+            if (
+                reserve_capacity_slots is None
+                and weekly_target is None
+                and self.recent_model_reserve_slots > 0
+            ):
+                # Recent models can use every position; old models cannot.
+                # Pack recent work into reserved positions first, then the
+                # account nearest its old-model threshold. This preserves
+                # shared positions for later old candidates, regardless of
+                # candidate arrival order. Keep largest-headroom/round-robin
+                # balancing among accounts with the same shared capacity.
+                shared_slots = {
+                    index: max(0, remaining - self.recent_model_reserve_slots)
+                    for index, remaining in usable.items()
+                }
+                least_shared = min(shared_slots.values())
+                usable = {
+                    index: remaining for index, remaining in usable.items()
+                    if shared_slots[index] == least_shared
+                }
             best_remaining = max(usable.values())
             tied = [index for index, remaining in usable.items() if remaining == best_remaining]
             selected_index = min(