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.

README.md

@@ -311,6 +311,16 @@ tries the next account. It does not lower the persisted capacity using a cached
 active count: another worker may have filled the queue since that count was
 read. Downward capacity learning remains confined to weekly calibration.
 
+Account allocation is age-aware: recent models first use accounts with only
+reserved positions left, then the account with the fewest shared positions
+remaining. Old models continue to use the account with the most eligible space.
+Largest-headroom and rotating tie-breaks balance otherwise equivalent accounts;
+weekly calibration remains pinned to its target account. For example, with
+accounts at 94/100 and 95/100 and five recent-only positions each, assigning a
+recent model to the second account preserves the first account's last old-model
+position. A recent-then-old pair can both be accepted rather than stranding the
+old model after the recent model takes its only eligible position.
+
 Model verification results are reused for 15 minutes across poll cycles, and a
 locally failed model/GPU pair cools down for 24 hours instead of being excluded
 forever. The `[scan]` lines show every expansion stage, while `[daily] wave_done`