feat(spells): Phase E.5 CastSpellRequest + Spellbook/enchantment state

Completes the client-side spell loop on top of Phase F.1. Player can
send cast requests; spellbook + active-enchantment state mirrors what
the server broadcasts.

Wire layer:
- CastSpellRequest (C→S, inside 0xF7B1 GameAction):
  - BuildUntargeted (0x0048): self-buffs, recalls, heal-self — 16 bytes.
  - BuildTargeted (0x004A): projectile attacks, target buffs/debuffs — 20 bytes.
- GameEvents parsers added:
  - 0x02C1 MagicUpdateSpell: spell-id → spellbook.
  - 0x01A8 MagicRemoveSpell.
  - 0x02C2 MagicUpdateEnchantment: spellId + layerId + duration + casterGuid
    (summary head; full stat-mod body deferred).
  - 0x02C3 MagicRemoveEnchantment: (layerId, spellId).
  - 0x02C7 MagicDispelEnchantment: same shape.

Core layer:
- Spellbook: learned-spell set + active-enchantment-by-layer dict
  with events (SpellLearned, SpellForgotten, EnchantmentAdded,
  EnchantmentRemoved). Duplicate learn is idempotent. Same-layer
  add refreshes duration. Purge fires per-record remove for UI
  cleanup.
- ActiveEnchantmentRecord: (SpellId, LayerId, Duration, CasterGuid).

Tests (10 new):
- CastSpellRequest untargeted (16 bytes) + targeted (20 bytes) wire encoding.
- GameEvents: MagicUpdateSpell, MagicUpdateEnchantment,
  MagicRemoveEnchantment round-trip.
- Spellbook: learn idempotent, forget, add/refresh enchantment,
  remove fires event, purge-all clears + fires per-record.

Build green, 555 tests pass (up from 544).

Ref: r01 §2 (wire casts), §3 (cast state machine), §5 (stacking rules).
Ref: r08 §3 opcodes 0x0048/0x004A, §4 opcodes 0x01A8/0x02C1-0x02C8.

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

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

@@ -0,0 +1,64 @@
+using System.Buffers.Binary;
+
+namespace AcDream.Core.Net.Messages;
+
+/// <summary>
+/// Outbound <c>Magic_CastSpell</c> GameActions — targeted and untargeted
+/// variants. From r01 §2 + r08 §3 opcodes 0x0048 / 0x004A.
+///
+/// <para>
+/// Wire layouts (inside the <c>0xF7B1</c> GameAction envelope):
+/// </para>
+///
+/// <para>
+/// Untargeted (<c>0x0048</c>):
+/// <code>
+///   u32 0xF7B1
+///   u32 gameActionSequence
+///   u32 0x0048
+///   u32 spellId
+/// </code>
+/// </para>
+///
+/// <para>
+/// Targeted (<c>0x004A</c>):
+/// <code>
+///   u32 0xF7B1
+///   u32 gameActionSequence
+///   u32 0x004A
+///   u32 targetGuid
+///   u32 spellId
+/// </code>
+/// </para>
+/// </summary>
+public static class CastSpellRequest
+{
+    public const uint GameActionEnvelope = 0xF7B1u;
+    public const uint UntargetedSubOpcode = 0x0048u;
+    public const uint TargetedSubOpcode   = 0x004Au;
+
+    /// <summary>Build an untargeted cast (buff self, recall, self-heal, etc).</summary>
+    public static byte[] BuildUntargeted(uint gameActionSequence, uint spellId)
+    {
+        byte[] body = new byte[16];
+        BinaryPrimitives.WriteUInt32LittleEndian(body,            GameActionEnvelope);
+        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(4),  gameActionSequence);
+        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(8),  UntargetedSubOpcode);
+        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(12), spellId);
+        return body;
+    }
+
+    /// <summary>
+    /// Build a targeted cast (projectile attacks, target buffs, debuffs).
+    /// </summary>
+    public static byte[] BuildTargeted(uint gameActionSequence, uint targetGuid, uint spellId)
+    {
+        byte[] body = new byte[20];
+        BinaryPrimitives.WriteUInt32LittleEndian(body,            GameActionEnvelope);
+        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(4),  gameActionSequence);
+        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(8),  TargetedSubOpcode);
+        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(12), targetGuid);
+        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(16), spellId);
+        return body;
+    }
+}