feat(allegiance): Phase H.2 AllegianceRequests + AllegianceTree model

Client-side allegiance data model + outbound swear/break actions.
The inbound AllegianceUpdate blob (0x0020) is complex and is deferred;
the tree API here is designed so the handler can push nodes in when
the blob parser lands.

Wire layer:
- AllegianceRequests.BuildSwear (0x001D): single uint32 patronGuid.
- AllegianceRequests.BuildBreak (0x001E): single uint32 targetGuid
  (works for both breaking from patron and breaking away a vassal;
  the server picks behavior based on the relationship).

Core layer (AcDream.Core/Allegiance):
- AllegianceNode: Guid, Name, PatronGuid, Rank (clamped 0..10),
  VassalGuids list.
- AllegianceTree: Dictionary-backed, events on TreeChanged.
  - SetMonarch: registers the root (no patron).
  - UpsertNode: adds/refreshes + auto-inserts into parent's vassal list.
  - RemoveNode: removes from parent list too; descendants are left with
    dangling patron pointers for the UI to hide (next AllegianceUpdate
    refreshes).
  - GetAncestors: walks up to monarch, cycle-detected for defense.
  - GetDescendants: BFS-order flattening.
- AllegianceMath.ComputePassup: retail XP formula
  (50+22.5×loyalty)/291 × (1+RT/730×IG/720) × earned,
  clamped at 0.

Tests (11 new):
- Tree: SetMonarch fires TreeChanged, UpsertNode auto-populates parent
  vassal list, rank clamp at 10, RemoveNode cleans parent list,
  GetAncestors chain, cycle-safe walk, GetDescendants BFS order.
- Math: Passup known-value check (1000 XP, 10 loyalty, 100 RT/IG
  days → ~963 XP), negative clamp.
- Wire: Swear + Break byte-exact encoding.

Build green, 613 tests pass (up from 602).

Next: wire inbound AllegianceUpdate (0x0020) + AllegianceInfoResponse
(0x027C) handlers once the blob parser lands. Chat "Allegiance"
Turbine channel joining (r11 §2.1 step 9) layers on top of
Phase H.1 chat infrastructure.

Ref: r11 §1 (tree structure + rank cap), §2 (swear/break wire),
§3.2 (XP passup formula).

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

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

@@ -0,0 +1,57 @@
+using System.Buffers.Binary;
+
+namespace AcDream.Core.Net.Messages;
+
+/// <summary>
+/// Outbound allegiance GameActions. Both Swear and Break carry a
+/// single <c>uint32</c> target-guid payload inside the standard
+/// <c>0xF7B1</c> GameAction envelope.
+///
+/// <para>
+/// Wire layout (r11 §2.1 / §2.3):
+/// <code>
+///   u32 0xF7B1
+///   u32 gameActionSequence
+///   u32 subOpcode (0x001D or 0x001E)
+///   u32 targetGuid
+/// </code>
+/// </para>
+///
+/// <para>
+/// Server replies with <c>GameEventAllegianceUpdate</c> (0x0020) and
+/// <c>GameEventAllegianceAllegianceUpdateDone</c> (0x01C8) on success,
+/// or a <c>WeenieError</c> on failure (already sworn, already maxed
+/// vassals, target not online, etc — see r11 §2.1 for the full list).
+/// </para>
+/// </summary>
+public static class AllegianceRequests
+{
+    public const uint GameActionEnvelope = 0xF7B1u;
+    public const uint SwearOpcode = 0x001Du;
+    public const uint BreakOpcode = 0x001Eu;
+
+    /// <summary>Pledge yourself to the given patron.</summary>
+    public static byte[] BuildSwear(uint gameActionSequence, uint patronGuid)
+    {
+        return Build(gameActionSequence, SwearOpcode, patronGuid);
+    }
+
+    /// <summary>
+    /// Break your pledge to <paramref name="targetGuid"/>. Target can be
+    /// your patron (breaking from) OR your vassal (breaking them away).
+    /// </summary>
+    public static byte[] BuildBreak(uint gameActionSequence, uint targetGuid)
+    {
+        return Build(gameActionSequence, BreakOpcode, targetGuid);
+    }
+
+    private static byte[] Build(uint seq, uint sub, uint targetGuid)
+    {
+        byte[] body = new byte[16];
+        BinaryPrimitives.WriteUInt32LittleEndian(body,            GameActionEnvelope);
+        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(4),  seq);
+        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(8),  sub);
+        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(12), targetGuid);
+        return body;
+    }
+}