refactor: master views over BaseCreature's one master reference (#2674)

## Summary

After #2670, `BaseCreature` stores one master. Callers still reached it through three differently gated names plus hand-rolled combinations of the `Controlled`/`Summoned` flags. Each gate is now one named view:

| Member | Definition | Meaning |
|---|---|---|
| `Master` | the reference; its setter does the follower bookkeeping; serialized as before | whoever the creature answers to |
| `ControlMaster` | `Controlled ? Master : null` | the owner |
| `SummonMaster` | `Summoned ? Master : null` | the summoner |
| `GetMaster()` | `ControlMaster ?? SummonMaster` | who answers for the creature |

`ControlMaster` and `SummonMaster` stay settable so custom code keeps compiling: both setters assign `Master`. There is no guard, so a write through either one can read back as null until the matching flag is set.

An enraged creature is the one shape with a master and neither flag. It keeps its meer through `Master`, while `GetMaster()` stays null, so the meer never answers for it (notoriety, kill credit).

Readers move onto the view they mean:
- "Owner or summoner" pairs and `ControlMaster ?? SummonMaster` collapse into `GetMaster()`: pack instinct, familiars, golem, house access, mounts, pack horse, Solen friendship, spell target and guild checks, and the `[` pet command.
- Checks such as `Controlled && ControlMaster == from` drop the flag the view already applies. Checks that don't reduce keep it: `Controlled && ControlMaster != from`, a possibly-null `from`, and `MageAI.CanDispel`'s `Summoned &&`.

**No behavior change, save format unchanged** (the schema generator produces no diff). Edge cases for custom code and shard operators:
- `SummonMaster = x` on a plain pet now replaces its owner (#2670 made that a no-op), and `ControlMaster = null` on an uncontrolled summon now clears its caster (it did nothing before).
- `SpellHelper.GetGuildFor` no longer falls back to an enraged creature's meer. Meer mages have no guild.

## Docs

- `dev-docs/content-patterns.md` § Masters: which view answers which question, how to set a master, and which flag checks are redundant.
- `dev-docs/runuo-migration-docs/09-items-mobiles-creatures.md` § Masters: RunUO's two-field reads mapped onto the views, plus the gotcha for creatures that set `SummonMaster` without `Summoned` (the write compiles, the read now returns null; use `Master`).
- The `modernuo-content-patterns` and `migrate-items-mobiles` skills point at both.

## Test plan

- [x] `MasterViewsTests`: all four views across wild, pet, controlled summon, uncontrolled summon and enraged shapes; reassigning `Master` moves follower slots; `SetControlMaster(null)` keeps an uncontrolled summon's caster; deleting a summon returns the caster's slots
- [x] Serialization tests updated to the new views (round-trip, legacy v22, v23 migration)
- [x] UOContent.Tests (1178 passed, 2 skipped) and Server.Tests (907) green

Projects/UOContent/Mobiles/AI/BaseAI/BaseAI.cs

@@ -144,7 +144,7 @@ public abstract partial class BaseAI
         }
 
         // Only a creature somebody can command has been told anything; wild creatures rest on None.
-        var toldToStandDown = Mobile.Controlled && Mobile.ControlMaster != null && Mobile.Commandable &&
+        var toldToStandDown = Mobile.ControlMaster != null && Mobile.Commandable &&
                               IsStandDownOrder(Mobile.ControlOrder);
 
         if (Mobile.StandsDownOnCommand && toldToStandDown)
@@ -1049,15 +1049,15 @@ public abstract partial class BaseAI
     private bool IsInvalidSummonTarget(Mobile m, BaseCreature bc)
     {
         // A summon whose caster was deleted before a reload comes back with no master.
-        if (Core.AOS && bc?.Summoned == true &&
-            (bc.SummonMaster == Mobile || bc.SummonMaster is { Player: false } summonMaster && IsHostile(summonMaster)))
+        if (Core.AOS &&
+            (bc?.SummonMaster == Mobile || bc?.SummonMaster is { Player: false } summonMaster && IsHostile(summonMaster)))
         {
             return true;
         }
 
         var master = Mobile.SummonMaster;
 
-        if (!Mobile.Summoned || master == null)
+        if (master == null)
         {
             return false;
         }