fix(ai): BaseAI.OnAggressiveAction owns the retaliation policy; familiars refuse cleanly (#2662)

> [!IMPORTANT]
> **Breaking change to API to fix NPC AI issues**

## Symptom

`FamiliarAITests.HiddenCaster_FamiliarRefusesRetaliation` fails on main at `Assert.False(wolf.Warmode)`: the familiar's `Combatant` stays null as intended, but it is left in war stance. In-game the familiar of a hidden caster flips into warmode for a tick when struck, until `FamiliarAI.StandDown()` clears it on the next think.

CI never saw it because the familiar tests are `SkippableFact` (they need client tiledata).

## Root cause

#2644 and #2645 landed a day apart and are individually correct but conflict at one seam.

Since #2645, a familiar (`Commandable == false`) is no longer "told to stand down", so `BaseCreature.AggressiveAction` continues past `AIObject.OnAggressiveAction(aggressor)` to its generic fallback:

```cs
else if (Combatant == null && !BardPacified)
{
    Warmode = true;
    Combatant = aggressor;
}
```

`FamiliarAI.OnAggressiveAction` had already declined (caster hidden), but the hook was `void`, so the creature could not tell. The fallback raised `Warmode` before `BaseFamiliar.Combatant` vetoed the target.

## Fix

This is the follow-up agreed in #2645: the retaliation policy moves into a virtual `BaseAI.OnAggressiveAction(aggressor) -> bool`.

- **`BaseAI.OnAggressiveAction`** now holds the stand-down verdict, the closer-aggressor swap (`PreferCloserAggressor`), and the `IssueOrder(Attack)` vs `Combatant =` decision. Returns `false` when the creature stands down.
- **`BaseCreature.AggressiveAction`** keeps only bookkeeping — `ControlMaster.Aggressors` entry, `StopFlee`, `ForceReacquire`, ethics `FinishShield` — keyed off the return, so an AI that declines is never second-guessed.
- **`FamiliarAI`** overrides the whole policy: no orders to stand down on, never fights for a hidden caster or a hidden aggressor.

Behaviour for every other AI is unchanged. Moving the fallback ahead of `StopFlee`/`ForceReacquire` is order-safe: both are pure field resets, `IssueAttack` does not `Think()`, and `OnCombatantChange` only derives `Warmode`.

Two commits: the first is the minimal `bool` hook that alone fixes the test, the second the policy move.

## Not changed (noted for later)

- `Warmode = true; Combatant = x;` (also in `FamiliarAI.Act`, `IssueAttack`) is redundant when the setter accepts — `BaseCreature.OnCombatantChange` derives `Warmode` — and is the dangling-warmode hazard whenever a `Combatant` setter vetoes.
- Since #2614 a stood-down pet skips the ethics `FinishShield`; RunUO ran it unconditionally. Preserved as-is.

## Tests

`HiddenCaster_FamiliarRefusesRetaliation` fails on main and passes here. Full `UOContent.Tests`: 1126 passed, 0 failed, 2 skipped (spawner fixture capture + benchmark).

dev-docs/claude-skills/migrate-from-runuo/migrate-items-mobiles.md

@@ -31,6 +31,7 @@ description: >
 - Expression-bodied overrides: `public override int Meat { get { return 1; } }` -> `public override int Meat => 1;`
 - AI movement calls lose the `run` flag: `MoveTo(m, true, range)` -> `MoveTo(m, range)` (also `WalkMobileRange`, `ApproachTarget`, `MoveToPoint`, `PathFollower.Follow`); the Running bit is derived from step pace -> `dev-docs/runuo-migration-docs/09-items-mobiles-creatures.md` § AI Movement
 - `AcquireOnApproach` (bool) -> `AcquireOnApproachDelay` (TimeSpan; `Zero` = old instant behavior) -> same doc § Target Acquisition
+- `BaseAI.OnAggressiveAction(aggressor)` is `bool`, not `void`: the AI owns the retaliation policy (`BaseCreature` no longer sets `Combatant` after it); an early `return;` that refused a fight -> `return false;`, otherwise `return true;`; swap-only overrides call `PreferCloserAggressor(aggressor)` instead of `base` -> same doc § Retaliation
 - `DamageEntries` is an inline `ref readonly ValueLinkList<DamageEntry>`, not a `List`: indexer/`Add`/`Remove`/`Clear` -> `foreach` / `.ByDescending()` (needs `using Server.Collections;`) and `ClearDamageEntries()`; `GetLootingRights` takes it by `in` -> same doc § Damage Entries
 
 ## Anti-Patterns