perf(ai): make IsEnemy the single authority for Honor/Ethereal Voyage (#2655)

## Summary
Profiling with thousands of creatures in range of each other showed `BaseAI.IsInvalidFactionTarget` checking Ethereal Voyage and active Honor on every acquisition candidate and then calling `BaseCreature.IsEnemy`, which checked both again. This makes `IsEnemy` the single authority and removes the duplicate per-pair work.

## Changes
- **`BaseCreature.IsEnemy`**: the Ethereal Voyage check sat below the `m is not BaseCreature` early return, so it never reached players — the only mobiles that cast it. It is hoisted next to the Honor veto. `GetMaster()` was called three times per creature pair (inside `Ethics.Player.Find(m, true)` and twice at the bottom); it is now computed once.
- **`BaseAI.IsInvalidFactionTarget`**: reduced to `IsFriend` / `IsEnemy` / `CanBeHarmful`. The removed `Combatant != m` Honor exception was dead code — `IsEnemy` vetoed Honor unconditionally on the following line.
- **`Ethics.Player.Find`** / **`TransformationSpellHelper.GetContext`**: small simplifications on the same path.

## Behavior
- `ShouldAcquireOnApproach` and `OnAggressiveAction` only consult `IsEnemy`, so movement-triggered acquisition now respects Ethereal Voyage for players (previously a player under Ethereal Voyage walking past a monster was still acquired on approach).
- `BaseFactionGuard.IsEnemy` does not call base, so faction guards no longer skip honoring/voyaging enemy-faction players. Accepted: both mechanics describe monsters, not guards.
- `HealerAI`/`BerserkAI`/`PredatorAI` (`bFacFriend`) callers are unaffected — `IsFriend` already required a `BaseCreature`, so players were excluded before these checks ran. `MilitiaFighter`/`MilitiaCanoneer` return `false` for all players, so they are unaffected too.

## Testing
- `dotnet build` clean.
- Pure predicate reorder; no new tests.

Projects/UOContent/Spells/Base/SpellHelper.cs

@@ -1340,12 +1340,7 @@ namespace Server.Spells
             context.Spell.RemoveEffect(m);
         }
 
-        public static TransformContext GetContext(Mobile m)
-        {
-            _table.TryGetValue(m, out var context);
-
-            return context;
-        }
+        public static TransformContext GetContext(Mobile m) => _table.GetValueOrDefault(m);
 
         public static bool UnderTransformation(Mobile m) => GetContext(m) != null;