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.App.Tests/UI/ItemInteractionControllerTests.cs

@@ -27,6 +27,7 @@ public sealed class ItemInteractionControllerTests
         public readonly List<uint> Drops = new();
         public readonly List<(uint Item, uint Amount)> SplitDrops = new();
         public readonly List<(uint Target, uint Item, uint Amount)> Gives = new();
+        public readonly List<(uint VendorGuid, uint ItemGuid, int Amount, uint AlternateCurrencyId)> Buys = new();
         public readonly List<string> Toasts = new();
         public readonly List<string> SystemMessages = new();
         public readonly List<CombatMode> CombatModeRequests = new();
@@ -94,7 +95,9 @@ public sealed class ItemInteractionControllerTests
                 },
                 combatState: Combat,
                 sendChangeCombatMode: CombatModeRequests.Add,
-                requestUse: requestUse);
+                requestUse: requestUse,
+                sendBuy: (vendorGuid, itemGuid, amount, alternateCurrencyId) =>
+                    Buys.Add((vendorGuid, itemGuid, amount, alternateCurrencyId)));
         }
 
         public ItemInteractionController Controller { get; }
@@ -2145,4 +2148,123 @@ public sealed class ItemInteractionControllerTests
         Assert.Equal(0, h.Controller.BusyCount);
         Assert.Equal(InteractionModeKind.None, h.Controller.InteractionState.Current.Kind);
     }
+
+    // ── Slice 6.3: TryBuy ───────────────────────────────────────────────
+
+    [Fact]
+    public void TryBuy_Succeeds_SendsBuyAndTakesTheSharedUseReservation()
+    {
+        var h = new Harness();
+
+        bool result = h.Controller.TryBuy(
+            vendorGuid: 0x40001000u,
+            itemGuid: 0x50002000u,
+            amount: 1,
+            alternateCurrencyId: 0u);
+
+        Assert.True(result);
+        Assert.Equal(
+            new[] { (0x40001000u, 0x50002000u, 1, 0u) },
+            h.Buys);
+        // BeginUseRequestReservation increments BusyCount synchronously,
+        // before/independent of any wire response -- this is what makes the
+        // Buy button disable immediately (research doc §A.4: "no second
+        // gate", the SAME BusyCount>0 check every other request rides).
+        Assert.Equal(1, h.Controller.BusyCount);
+    }
+
+    [Fact]
+    public void TryBuy_StackedQuantity_ForwardsTheExactAmount()
+    {
+        var h = new Harness();
+
+        Assert.True(h.Controller.TryBuy(0x40001000u, 0x50002001u, 25, 0u));
+
+        Assert.Equal(25, h.Buys.Single().Amount);
+    }
+
+    [Fact]
+    public void TryBuy_AlternateCurrencyVendor_ForwardsTheCurrencyWcid()
+    {
+        var h = new Harness();
+
+        Assert.True(h.Controller.TryBuy(0x40001000u, 0x50002000u, 1, 0x12345678u));
+
+        Assert.Equal(0x12345678u, h.Buys.Single().AlternateCurrencyId);
+    }
+
+    [Fact]
+    public void TryBuy_WhileAnotherRequestIsBusy_IsRejectedAndSendsNothing()
+    {
+        var h = new Harness();
+        h.Controller.IncrementBusyCount(); // simulates any other in-flight request
+
+        bool result = h.Controller.TryBuy(0x40001000u, 0x50002000u, 1, 0u);
+
+        Assert.False(result);
+        Assert.Empty(h.Buys);
+        Assert.Equal(1, h.Controller.BusyCount); // unchanged -- no second reservation taken
+    }
+
+    [Fact]
+    public void TryBuy_ASecondBuyWhileTheFirstIsInFlight_IsRejected()
+    {
+        var h = new Harness();
+        Assert.True(h.Controller.TryBuy(0x40001000u, 0x50002000u, 1, 0u));
+
+        bool second = h.Controller.TryBuy(0x40001000u, 0x50002001u, 1, 0u);
+
+        Assert.False(second);
+        Assert.Single(h.Buys);
+        Assert.Equal(1, h.Controller.BusyCount);
+    }
+
+    [Theory]
+    [InlineData(0u, 0x50002000u, 1)]
+    [InlineData(0x40001000u, 0u, 1)]
+    [InlineData(0x40001000u, 0x50002000u, 0)]
+    [InlineData(0x40001000u, 0x50002000u, -1)]
+    public void TryBuy_InvalidArguments_IsRejectedWithoutTakingAReservation(
+        uint vendorGuid, uint itemGuid, int amount)
+    {
+        var h = new Harness();
+
+        bool result = h.Controller.TryBuy(vendorGuid, itemGuid, amount, 0u);
+
+        Assert.False(result);
+        Assert.Empty(h.Buys);
+        Assert.Equal(0, h.Controller.BusyCount);
+    }
+
+    [Fact]
+    public void TryBuy_CompleteUse_ReleasesTheReservationAndReenablesFurtherRequests()
+    {
+        // Research doc §A.4: UseDone (0x01C7) is the completion signal for
+        // Buy, resolved through the SAME RuntimeInteractionTransactionState.
+        // CompleteUse the existing UseDone handler already calls -- no new
+        // completion plumbing needed on the receive side.
+        var h = new Harness();
+        Assert.True(h.Controller.TryBuy(0x40001000u, 0x50002000u, 1, 0u));
+        Assert.Equal(1, h.Controller.BusyCount);
+
+        h.Controller.CompleteUse(0);
+
+        Assert.Equal(0, h.Controller.BusyCount);
+        // The gate is free again -- a second Buy can now proceed.
+        Assert.True(h.Controller.TryBuy(0x40001000u, 0x50002001u, 1, 0u));
+        Assert.Equal(2, h.Buys.Count);
+    }
+
+    [Fact]
+    public void TryBuy_FailedUseDone_AlsoReleasesTheReservation()
+    {
+        // A.2's failure paths all still end in exactly one SendUseDoneEvent
+        // -- success or failure, the reservation resolves the same way.
+        var h = new Harness();
+        Assert.True(h.Controller.TryBuy(0x40001000u, 0x50002000u, 1, 0u));
+
+        h.Controller.CompleteUse(0x0009u); // an arbitrary nonzero WeenieError
+
+        Assert.Equal(0, h.Controller.BusyCount);
+    }
 }