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>

src/AcDream.Core.Net/Messages/CharacterActions.cs

@@ -0,0 +1,79 @@
+using System.Buffers.Binary;
+
+namespace AcDream.Core.Net.Messages;
+
+/// <summary>
+/// Outbound character-progression + mode-change GameActions. Grouped
+/// together because they share the same <c>0xF7B1</c> envelope +
+/// single-opcode dispatch pattern and all target the session's player
+/// (no target guid field).
+///
+/// <para>
+/// References: r08 §3 rows 0x0044 / 0x0045 / 0x0046 / 0x0047 / 0x0053.
+/// </para>
+/// </summary>
+public static class CharacterActions
+{
+    public const uint GameActionEnvelope = 0xF7B1u;
+
+    public const uint RaiseAttributeOpcode = 0x0045u;  // u32 attr, u64 xpSpent
+    public const uint RaiseVitalOpcode     = 0x0044u;  // u32 vital, u64 xpSpent
+    public const uint RaiseSkillOpcode     = 0x0046u;  // u32 skillId, u64 xpSpent
+    public const uint TrainSkillOpcode     = 0x0047u;  // u32 skillId, u32 credits
+    public const uint ChangeCombatModeOpcode = 0x0053u; // u32 combatMode
+
+    public enum CombatMode : uint
+    {
+        Undef = 0, NonCombat = 1, Melee = 2, Missile = 3, Magic = 4, Peaceful = 5,
+    }
+
+    /// <summary>Spend XP to raise an attribute (Strength, Endurance, etc).</summary>
+    public static byte[] BuildRaiseAttribute(uint seq, uint attrId, ulong xpSpent)
+        => BuildAttrOrVital(seq, RaiseAttributeOpcode, attrId, xpSpent);
+
+    /// <summary>Spend XP to raise a vital (Health, Stamina, Mana).</summary>
+    public static byte[] BuildRaiseVital(uint seq, uint vitalId, ulong xpSpent)
+        => BuildAttrOrVital(seq, RaiseVitalOpcode, vitalId, xpSpent);
+
+    /// <summary>Spend XP to raise a skill.</summary>
+    public static byte[] BuildRaiseSkill(uint seq, uint skillId, ulong xpSpent)
+        => BuildAttrOrVital(seq, RaiseSkillOpcode, skillId, xpSpent);
+
+    /// <summary>Spend skill credits to train (unlock) a skill.</summary>
+    public static byte[] BuildTrainSkill(uint seq, uint skillId, uint credits)
+    {
+        byte[] body = new byte[20];
+        BinaryPrimitives.WriteUInt32LittleEndian(body,            GameActionEnvelope);
+        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(4),  seq);
+        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(8),  TrainSkillOpcode);
+        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(12), skillId);
+        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(16), credits);
+        return body;
+    }
+
+    /// <summary>
+    /// Change combat mode. Peace ↔ Melee / Missile / Magic toggles that
+    /// switch stance + weapon ready-state. The animation + weapon
+    /// wield broadcasts are server-driven from this message.
+    /// </summary>
+    public static byte[] BuildChangeCombatMode(uint seq, CombatMode mode)
+    {
+        byte[] body = new byte[16];
+        BinaryPrimitives.WriteUInt32LittleEndian(body,            GameActionEnvelope);
+        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(4),  seq);
+        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(8),  ChangeCombatModeOpcode);
+        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(12), (uint)mode);
+        return body;
+    }
+
+    private static byte[] BuildAttrOrVital(uint seq, uint sub, uint id, ulong xp)
+    {
+        byte[] body = new byte[24];
+        BinaryPrimitives.WriteUInt32LittleEndian(body,            GameActionEnvelope);
+        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(4),  seq);
+        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(8),  sub);
+        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(12), id);
+        BinaryPrimitives.WriteUInt64LittleEndian(body.AsSpan(16), xp);
+        return body;
+    }
+}