feat(char): character progression actions — Raise / Train / CombatMode

Outbound GameActions for XP-spending + combat-mode-change. These
complete the wire surface for the character-sheet UI: the player
clicks "spend XP on Strength," the panel calls BuildRaiseAttribute,
the session sends it, the server responds with updated PlayerDescription
or PrivateUpdateAttribute GameEvents.

Wire layer:
- BuildRaiseAttribute (0x0045): attrId u32, xpSpent u64.
- BuildRaiseVital (0x0044): vitalId u32, xpSpent u64.
- BuildRaiseSkill (0x0046): skillId u32, xpSpent u64.
- BuildTrainSkill (0x0047): skillId u32, credits u32 (note: credits
  is u32 here, NOT u64 like the xpSpent variants).
- BuildChangeCombatMode (0x0053): mode enum as u32
  (Undef=0, NonCombat=1, Melee=2, Missile=3, Magic=4, Peaceful=5).

Tests (5 new): byte-exact encoding of each, including the Train/
Raise size difference due to u32 vs u64 payloads.

Build green, 621 tests pass (up from 616).

Ref: r08 §3 rows 0x0044 / 0x0045 / 0x0046 / 0x0047 / 0x0053.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/AcDream.Core.Net.Tests/Messages/CharacterActionsTests.cs

@@ -0,0 +1,60 @@
+using System;
+using System.Buffers.Binary;
+using AcDream.Core.Net.Messages;
+using Xunit;
+
+namespace AcDream.Core.Net.Tests.Messages;
+
+public sealed class CharacterActionsTests
+{
+    [Fact]
+    public void BuildRaiseAttribute_HasOpcode0x0045AndXp64()
+    {
+        byte[] body = CharacterActions.BuildRaiseAttribute(seq: 1, attrId: 5, xpSpent: 12345678);
+        Assert.Equal(24, body.Length);
+        Assert.Equal(CharacterActions.RaiseAttributeOpcode,
+            BinaryPrimitives.ReadUInt32LittleEndian(body.AsSpan(8)));
+        Assert.Equal(5u,
+            BinaryPrimitives.ReadUInt32LittleEndian(body.AsSpan(12)));
+        Assert.Equal(12345678u,
+            BinaryPrimitives.ReadUInt64LittleEndian(body.AsSpan(16)));
+    }
+
+    [Fact]
+    public void BuildRaiseVital_HasOpcode0x0044()
+    {
+        byte[] body = CharacterActions.BuildRaiseVital(seq: 1, vitalId: 1, xpSpent: 100);
+        Assert.Equal(CharacterActions.RaiseVitalOpcode,
+            BinaryPrimitives.ReadUInt32LittleEndian(body.AsSpan(8)));
+    }
+
+    [Fact]
+    public void BuildRaiseSkill_HasOpcode0x0046()
+    {
+        byte[] body = CharacterActions.BuildRaiseSkill(seq: 1, skillId: 8, xpSpent: 500);
+        Assert.Equal(CharacterActions.RaiseSkillOpcode,
+            BinaryPrimitives.ReadUInt32LittleEndian(body.AsSpan(8)));
+    }
+
+    [Fact]
+    public void BuildTrainSkill_U32CreditsNotU64()
+    {
+        byte[] body = CharacterActions.BuildTrainSkill(seq: 1, skillId: 8, credits: 4);
+        Assert.Equal(20, body.Length);  // 12 + 4 + 4 vs 16 for 4+8
+        Assert.Equal(CharacterActions.TrainSkillOpcode,
+            BinaryPrimitives.ReadUInt32LittleEndian(body.AsSpan(8)));
+        Assert.Equal(4u,
+            BinaryPrimitives.ReadUInt32LittleEndian(body.AsSpan(16)));
+    }
+
+    [Fact]
+    public void BuildChangeCombatMode_EnumSerialises()
+    {
+        byte[] body = CharacterActions.BuildChangeCombatMode(
+            seq: 1, CharacterActions.CombatMode.Melee);
+        Assert.Equal(CharacterActions.ChangeCombatModeOpcode,
+            BinaryPrimitives.ReadUInt32LittleEndian(body.AsSpan(8)));
+        Assert.Equal(2u,    // Melee = 2
+            BinaryPrimitives.ReadUInt32LittleEndian(body.AsSpan(12)));
+    }
+}