feat(skills): SkillEvents.SkillUsed for cross-assembly subscribers; InternalsVisibleTo ModernSpawner.Tests (#2636)

## Summary

Two small additive changes that an external content assembly (ModernSpawner) needs, as separable commits.

**1. `SkillEvents.SkillUsed`** (`Projects/UOContent/Skills/SkillEvents.cs`, namespace `Server.Misc`): a plain C# event `Action<Mobile, Skill, bool success>` raised once per skill attempt from each of the four `Mobile_SkillCheck*` handlers, with the handler's own result. Attempts the handler resolves without a roll (too difficult, no challenge) raise too, so a grandmaster's trivial success and a guaranteed combat roll are observable. Not raised when the mobile lacks the skill. Each handler keeps its logic in a private core method and raises on the way out, so there is exactly one raise per attempt and `CheckSkill` itself is unchanged.

- **Why a plain event and not a `[GeneratedEvent]`:** generated events are compile-time static dispatch inside the UOContent compilation, so a subscriber in another assembly cannot use `[OnEvent]`. Shape follows `HelpEvents`.
- **Why "used", not "gained":** this is the XmlSpawner skill-trigger semantic (it wrapped the same four handlers and passed their result as `success`; its grammar was `Skill[+/-]` for success-only or failure-only). Gains are already observable through the existing skill-change notification on `Mobile`.
- **Cost:** one delegate null-check per attempt when nothing is subscribed; no boxing, no closure, no allocation. The handlers sit on the combat swing path.
- **Exception contract:** subscriber exceptions propagate, matching `EventSink`/`HelpEvents`; no try/catch by design.

**2. `InternalsVisibleTo("ModernSpawner.Tests")`** on `Server.csproj`, beside the existing `Server.Tests`/`UOContent.Tests` entries, so an external test host can seed `Core._now` the way the engine's own test initializers do. Separable; a public test seam on `Core` would serve the same need without naming a downstream assembly.

## Open question

The payload is the `Skill` object plus a positional `bool`. A `readonly struct` args type passed `in` would leave room to add `chance` or the target later without breaking subscribers. Happy to change before merge.

## Test plan

- [x] `UOContent.Tests`: 4 tests — a rolled attempt raises once with the returned outcome; each short-circuit path (no challenge, too difficult, on both the direct and value-window handlers) raises with the handler's result; a direct `CheckSkill` call does not raise; no subscriber does not throw. Full suite green.
- [x] `Server` and `UOContent` build clean with `TreatWarningsAsErrors`.
- [ ] CI

Projects/Server/Server.csproj

@@ -50,5 +50,8 @@
         <AssemblyAttribute Include="System.Runtime.CompilerServices.InternalsVisibleTo">
             <_Parameter1>UOContent.Tests</_Parameter1>
         </AssemblyAttribute>
+        <AssemblyAttribute Include="System.Runtime.CompilerServices.InternalsVisibleTo">
+            <_Parameter1>ModernSpawner.Tests</_Parameter1>
+        </AssemblyAttribute>
     </ItemGroup>
 </Project>

Projects/UOContent.Tests/Tests/Skills/SkillEventsTests.cs

@@ -0,0 +1,124 @@
+using Server;
+using Server.Misc;
+using Xunit;
+
+namespace UOContent.Tests;
+
+[Collection("Sequential UOContent Tests")]
+public class SkillEventsTests
+{
+    private sealed class Recorder
+    {
+        public Mobile From;
+        public Skill Skill;
+        public bool Success;
+        public int Calls;
+
+        public void Handle(Mobile from, Skill skill, bool success)
+        {
+            From = from;
+            Skill = skill;
+            Success = success;
+            Calls++;
+        }
+    }
+
+    [Fact]
+    public void DirectTarget_RolledAttempt_RaisesOnceWithTheReturnedOutcome()
+    {
+        var from = new Mobile();
+        var skill = from.Skills[SkillName.Mining];
+        var recorder = new Recorder();
+
+        SkillEvents.SkillUsed += recorder.Handle;
+        try
+        {
+            var rolled = SkillCheck.Mobile_SkillCheckDirectTarget(from, SkillName.Mining, null, 0.5);
+            Assert.Equal(1, recorder.Calls);
+            Assert.Same(from, recorder.From);
+            Assert.Same(skill, recorder.Skill);
+            Assert.Equal(rolled, recorder.Success);
+
+            Assert.False(SkillCheck.Mobile_SkillCheckDirectTarget(from, SkillName.Mining, null, 0.0));
+            Assert.Equal(2, recorder.Calls);
+            Assert.False(recorder.Success);
+        }
+        finally
+        {
+            SkillEvents.SkillUsed -= recorder.Handle;
+            from.Delete();
+        }
+    }
+
+    [Fact]
+    public void ShortCircuits_StillRaise_WithTheHandlerOutcome()
+    {
+        var from = new Mobile();
+        var recorder = new Recorder();
+
+        SkillEvents.SkillUsed += recorder.Handle;
+        try
+        {
+            Assert.True(SkillCheck.Mobile_SkillCheckDirectLocation(from, SkillName.Mining, 1.0));
+            Assert.Equal(1, recorder.Calls);
+            Assert.True(recorder.Success);
+
+            Assert.False(SkillCheck.Mobile_SkillCheckDirectTarget(from, SkillName.Mining, null, -0.1));
+            Assert.Equal(2, recorder.Calls);
+            Assert.False(recorder.Success);
+
+            Assert.False(SkillCheck.Mobile_SkillCheckLocation(from, SkillName.Mining, 50.0, 100.0));
+            Assert.Equal(3, recorder.Calls);
+            Assert.False(recorder.Success);
+
+            Assert.True(SkillCheck.Mobile_SkillCheckTarget(from, SkillName.Mining, null, 0.0, 0.0));
+            Assert.Equal(4, recorder.Calls);
+            Assert.True(recorder.Success);
+        }
+        finally
+        {
+            SkillEvents.SkillUsed -= recorder.Handle;
+            from.Delete();
+        }
+    }
+
+    [Fact]
+    public void CheckSkill_Direct_DoesNotRaise()
+    {
+        var from = new Mobile();
+        var skill = from.Skills[SkillName.Mining];
+        var recorder = new Recorder();
+
+        SkillEvents.SkillUsed += recorder.Handle;
+        try
+        {
+            SkillCheck.CheckSkill(from, skill, null, 1.0);
+            Assert.Equal(0, recorder.Calls);
+        }
+        finally
+        {
+            SkillEvents.SkillUsed -= recorder.Handle;
+            from.Delete();
+        }
+    }
+
+    [Fact]
+    public void NoSubscriber_DoesNotThrow()
+    {
+        var from = new Mobile();
+        var recorder = new Recorder();
+
+        try
+        {
+            SkillEvents.SkillUsed += recorder.Handle;
+            SkillEvents.SkillUsed -= recorder.Handle;
+
+            Assert.True(SkillCheck.Mobile_SkillCheckDirectLocation(from, SkillName.Mining, 1.0));
+            Assert.Equal(0, recorder.Calls);
+        }
+        finally
+        {
+            from.Delete();
+        }
+    }
+}

Projects/UOContent/Skills/SkillCheck.cs

@@ -48,6 +48,13 @@ public static class SkillCheck
             return false;
         }
 
+        var success = CheckLocation(from, skill, minSkill, maxSkill);
+        SkillEvents.InvokeSkillUsed(from, skill, success);
+        return success;
+    }
+
+    private static bool CheckLocation(Mobile from, Skill skill, double minSkill, double maxSkill)
+    {
         var value = skill.Value;
 
         if (value < minSkill)
@@ -76,6 +83,13 @@ public static class SkillCheck
             return false;
         }
 
+        var success = CheckDirectLocation(from, skill, chance);
+        SkillEvents.InvokeSkillUsed(from, skill, success);
+        return success;
+    }
+
+    private static bool CheckDirectLocation(Mobile from, Skill skill, double chance)
+    {
         if (chance < 0.0)
         {
             return false; // Too difficult
@@ -156,6 +170,13 @@ public static class SkillCheck
             return false;
         }
 
+        var success = CheckTarget(from, skill, target, minSkill, maxSkill);
+        SkillEvents.InvokeSkillUsed(from, skill, success);
+        return success;
+    }
+
+    private static bool CheckTarget(Mobile from, Skill skill, object target, double minSkill, double maxSkill)
+    {
         var value = skill.Value;
 
         if (value < minSkill)
@@ -182,6 +203,13 @@ public static class SkillCheck
             return false;
         }
 
+        var success = CheckDirectTarget(from, skill, target, chance);
+        SkillEvents.InvokeSkillUsed(from, skill, success);
+        return success;
+    }
+
+    private static bool CheckDirectTarget(Mobile from, Skill skill, object target, double chance)
+    {
         if (chance < 0.0)
         {
             return false; // Too difficult

Projects/UOContent/Skills/SkillEvents.cs

@@ -0,0 +1,23 @@
+using System;
+using System.Runtime.CompilerServices;
+
+namespace Server.Misc;
+
+/// <summary>
+/// Skill system events. Plain C# events so other assemblies can subscribe; generated events cannot be
+/// subscribed across assemblies.
+/// </summary>
+public static class SkillEvents
+{
+    /// <summary>
+    /// Raised once per skill attempt from the four <c>Mobile_SkillCheck*</c> handlers with the attempt's
+    /// outcome, including attempts the handler resolves without a roll (too difficult, no challenge).
+    /// Not raised when the mobile lacks the skill. Fires for every <see cref="Mobile" />, including
+    /// creatures. Subscribers must not block or allocate.
+    /// </summary>
+    public static event Action<Mobile, Skill, bool> SkillUsed;
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static void InvokeSkillUsed(Mobile from, Skill skill, bool success) =>
+        SkillUsed?.Invoke(from, skill, success);
+}

dev-docs/events.md

@@ -242,6 +242,17 @@ public static void HandlePlayerLogin(PlayerMobile player)
 
 ---
 
+## Static Content Events
+
+Generated events dispatch statically inside `UOContent`; content that other assemblies must observe exposes a
+plain `static event` instead (shape: `Projects/UOContent/Engines/Help/HelpEvents.cs`).
+
+- `SkillEvents.SkillUsed` -- `Action<Mobile, Skill, bool success>`, raised once per skill attempt from the
+  four `Mobile_SkillCheck*` handlers with the attempt's outcome, including attempts resolved without a roll
+  (too difficult, no challenge). Not raised when the mobile lacks the skill. Fires for every `Mobile`.
+
+---
+
 ## Event Args Pooling Pattern
 
 Some EventArgs use object pooling to avoid allocation in hot paths: