## Summary
`BaseCreature` stored the pet owner (`ControlMaster`) and the summoner (`SummonMaster`) in two fields. Every flow that manages pets kept them in step by hand: summon, transfer, stable, claim, ball of summoning and GM obey. They now share one `_master` field, and the two properties read it through the Controlled and Summoned flags:
```csharp
ControlMaster => Controlled ? master : null
SummonMaster => Controlled && !Summoned ? null : master
```
Every existing reader keeps its meaning:
- `ControlMaster` is still "the owner".
- `SummonMaster` is still "the summoner, controlled or not".
Blade spirits, energy vortexes, clones, undead minions and enraged creatures (a summon master without `Summoned`) never become visible through `ControlMaster`, so the ungated readers (ML notoriety, aggressor entries and similar) behave as before.
- Setting `SummonMaster` on a plain pet does nothing, instead of quietly replacing the owner.
- `SetControlMaster` now keeps both masters in step, so the hand-written `SummonMaster` copies are removed from ball of summoning, GM obey, pet transfer, auto-stable and claim, the animal trainer's stable and claim, and jail stabling.
## Serialization
BaseCreature moves to v24 with one serialized master (slot 15); the old `SummonMaster` slot is dropped.
- `MigrateFrom(V23Content)` resolves absent fields through the same default-value methods the live reader uses.
- A save that carried both masters (a summon traded before ownership followed transfers) resolves to the owner. Legacy pre-codegen streams follow the same rule.
Requires `ModernUO.Serialization.Generator` 4.2.0 (#2671). `UOContent.Tests` was still on 4.1.0 and is bumped to match.
## Test plan
- [x] `MasterShapes_RoundTrip`: controlled pet, controlled summon, energy vortex, enraged creature
- [x] `SummonMaster_OnPlainPet_KeepsTheOwner`: owner and follower counts unchanged
- [x] `LegacyV22Stream_DifferingMasters_OwnerWins`
- [x] `V23Stream_MigratesToOneMaster_AndDefaultsAbsentFields`: both master slots -> owner; flags past bit 32; absent fields take class defaults
- [x] UOContent.Tests (1168 passed, 2 skipped) and Server.Tests (907) green
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