feat(vendor): Slice 6 buy arc — shop items are real objects, vendor selection is THE selection, and Buy works (0x005F)

Three ordered pieces in one landing (the shared controller/composition
files carry all three; the internal order was 6.1 -> 6.2 -> 6.3):

6.1 VendorShopItemMaterializer diff-merges the shop list into the live
ClientObjectTable on VendorState transitions (so client-local close and
session teardown retire the entries too) and never claims a guid it did
not add — ACE's UniqueItemsForSale can re-list a guid a player once
held (AP-163 files the collision-skip; no retail counterpart traced).
Right-click examine on shop items now routes through the ordinary
appraisal path — the 5.4 F7c blocker dissolves with the table entries.

6.2 SelectionChangeSource.Vendor: row clicks, auto-select, and examine
all flow through the canonical SelectionState; the status bar and the
existing byte-faithful StackSplitQuantityState slider light up
unmodified. VendorSplitPolicy is the single 0xDC41CB0 mask owner; the
slider VALUE seeds to 1 for exempt items while maxSplitSize keeps the
stack (the splitSize/maxSplitSize distinction, research §B.3).
Selection clears at retail's actual site — VendorItemsUI::RemoveFromShop
(pc:202848), not a CloseVendor-level clear that does not exist.

6.3 BuildBuy (0x005F): vendorGuid, count, (i32 amount, u32 guid) pairs,
and the trailing alternateCurrencyId the REAL client sends
(CM_Vendor::Event_Buy pc:689288) though ACE's reader ignores it.
TryBuy rides the EXISTING J5.2 one-request-at-a-time reservation and
completes on UseDone; the Buy button disables while a request is in
flight. The reconciliation round-trip (money property update, inventory
CreateObject, ApproachVendor refresh -> panel rebuild) is proven by a
synthetic-inbound test against existing machinery — no new owner.

Register: AP-161 narrowed (selection + examine residuals close;
staging/Sell remain; double-click-to-buy confirmed ABSENT from retail
with negative evidence cited — we match retail). AP-162 files the
conscious no-client-side-affordability-precheck deferral.

Clean-room complete solution: 11,368 passed / 4 skipped / 0 failed.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

tests/AcDream.Core.Net.Tests/WorldSessionVendorTests.cs

@@ -0,0 +1,64 @@
+using System.Net;
+using AcDream.Core.Net;
+using AcDream.Core.Net.Messages;
+
+namespace AcDream.Core.Net.Tests;
+
+/// <summary>
+/// Slice 6.3 — verifies <see cref="WorldSession.SendBuy"/> produces the same
+/// wire bytes <see cref="VendorRequests.BuildBuy"/> does directly, using a
+/// sequence number drawn from <see cref="WorldSession.NextGameActionSequence"/>.
+/// Mirrors <c>WorldSessionChatTests</c>'s <c>GameActionCapture</c> test seam.
+/// </summary>
+public sealed class WorldSessionVendorTests
+{
+    private static WorldSession NewSession()
+    {
+        var ep = new IPEndPoint(IPAddress.Loopback, 65001);
+        return new WorldSession(ep);
+    }
+
+    [Fact]
+    public void SendBuy_EmitsBytesIdenticalToVendorRequestsBuildBuy()
+    {
+        using var session = NewSession();
+        byte[]? captured = null;
+        session.GameActionCapture = body => captured = body;
+
+        session.SendBuy(
+            vendorGuid: 0x40001000u,
+            itemGuid: 0x50002000u,
+            amount: 3,
+            alternateCurrencyId: 0u);
+
+        byte[] expected = VendorRequests.BuildBuy(
+            gameActionSequence: 1,
+            vendorGuid: 0x40001000u,
+            amount: 3,
+            itemGuid: 0x50002000u,
+            alternateCurrencyId: 0u);
+
+        Assert.NotNull(captured);
+        Assert.Equal(expected, captured);
+    }
+
+    [Fact]
+    public void SendBuy_IncrementsTheSharedGameActionSequenceLikeEveryOtherSend()
+    {
+        using var session = NewSession();
+        byte[]? captured = null;
+        session.GameActionCapture = body => captured = body;
+
+        session.SendTalk("first"); // consumes sequence 1
+        session.SendBuy(0x40001000u, 0x50002000u, 1, 0u); // should be sequence 2
+
+        byte[] expected = VendorRequests.BuildBuy(
+            gameActionSequence: 2,
+            vendorGuid: 0x40001000u,
+            amount: 1,
+            itemGuid: 0x50002000u,
+            alternateCurrencyId: 0u);
+
+        Assert.Equal(expected, captured);
+    }
+}