Stacked on #2716. Retarget to `main` after it merges.
## Problem
Mobiles a champion summons mid-fight aren't tracked by anything:
| Summoner | Minions |
|---|---|
| LordOaks | Silvani queen |
| `SummonPixiesCounter` (LordOaks, Silvani, Twaulo) | 3-6 pixies per trigger |
| Barracoon | 3-6 ratmen per trigger |
`ChampionSpawn.DeleteCreatures` (called on expiry and when the spawn is deleted) clears only `Creatures` and the champion. These minions survive every reset and stay in the save. I surveyed every content creature that creates other mobiles; the others already clean up after themselves: Meraktus deletes its followers, Harrower kills its tentacles, PlagueSpawn and the Enraged animals delete themselves when their owner is gone, and BogThing eats its boglings.
## Changes
- **`BaseCreature.SpawnMinion(minion, target, location)`:** sets the team, places the minion and sets its combatant. Ownership: if the summoner is a champion spawn's champion, one of its creatures, or one of its minions (found through the spawn region), the minion joins that spawn. It refuses, and deletes the minion, if the summoner isn't on a real map. `SummonPixiesCounter` read `source.Map` even though `MonsterAbility.Trigger` notes it can fire after the creature has left the world.
- **`ChampionSpawn.Minions`:** a new serialized list (v12, `MigrateFrom(V11Content)`) kept apart from `Creatures`:
- killing a minion never counts toward a level, so a leftover summoner can't feed a restarted spawn;
- `DeleteCreatures` deletes minions along with the creatures, which matches when the spawn is deactivated (`ExpireCreatures`, 10 minutes after `Stop`) or deleted.
- **Callers:** LordOaks, `SummonPixiesCounter` and Barracoon spawn through `SpawnMinion`. A summoner that isn't part of a spawn (a GM `[add]`) still makes standalone minions, as today.
## Testing
- `ChampionMinionTests`:
- the champion's queen is listed in `Minions` and deleted by `DeleteCreatures`;
- a minion's minion also belongs to the spawn;
- a Lord Oaks in the region that isn't the spawn's champion keeps a standalone queen.
- UOContent.Tests: 1237 passed, 2 skipped. The Application build is clean.
69ff5f3940