refactor: one master reference behind ControlMaster and SummonMaster (#2670)

## 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

Projects/UOContent/Systems/JailSystem/JailSystem.cs

@@ -144,7 +144,6 @@ public class JailSystem : GenericPersistence
                     bc.Internalize();
 
                     bc.SetControlMaster(null);
-                    bc.SummonMaster = null;
 
                     bc.IsStabled = true;
                     bc.StabledBy = from;