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/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();
+        }
+    }
+}