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>

src/AcDream.App/UI/RetailUiRuntime.cs

@@ -112,7 +112,11 @@ public sealed record ToolbarRuntimeBindings(
     Action<uint> SendQueryHealth,
     Action<uint> SendQueryItemMana,
     Func<uint> PlayerGuid,
-    Action<uint, uint, int>? SendPutItemInContainer);
+    Action<uint, uint, int>? SendPutItemInContainer,
+    // Slice 6.2: composed at InteractionRetainedUiComposition from
+    // d.Inventory.Vendor + d.Inventory.Objects — SelectedObjectController's
+    // vendor-owned split-exempt-seed predicate (research doc §C.1).
+    Func<uint, bool> IsVendorSplitExempt);
 
 public sealed record CharacterRuntimeBindings(CharacterSheetProvider Provider);
 
@@ -170,7 +174,15 @@ public sealed record AppraisalRuntimeBindings(
 
 public sealed record VendorRuntimeBindings(
     VendorState State,
-    Func<ItemType, uint, uint, uint, uint, uint> ResolveIcon);
+    Func<ItemType, uint, uint, uint, uint, uint> ResolveIcon,
+    // Slice 6.1: shared examine routing — VendorUiController.ExamineItem
+    // calls ItemInteraction.ExamineSelectedOrEnterMode the same way
+    // ExternalContainerRuntimeBindings.ItemInteraction's consumer does.
+    ItemInteractionController ItemInteraction,
+    // Slice 6.2: the canonical selection authority — every sibling binding
+    // record (Radar/Magic/Toolbar/Inventory/ExternalContainer) already
+    // carries this; Vendor was the one outlier (research doc §C.1).
+    SelectionState Selection);
 
 public sealed record RetailUiRuntimeBindings(
     UiHost Host,
@@ -763,7 +775,8 @@ public sealed class RetailUiRuntime : IDisposable
             _bindings.Assets.DefaultFont,
             StackSplitQuantity,
             handler => b.Objects.ObjectUpdated += handler,
-            handler => b.Objects.ObjectUpdated -= handler);
+            handler => b.Objects.ObjectUpdated -= handler,
+            b.IsVendorSplitExempt);
 
         UiElement root = layout.Root;
         RetailWindowHandle handle = RetailWindowFrame.Mount(
@@ -1994,6 +2007,9 @@ public sealed class RetailUiRuntime : IDisposable
             b.ResolveIcon,
             _bindings.Inventory.Objects,
             _bindings.Inventory.PlayerGuid,
+            b.ItemInteraction,
+            b.Selection,
+            StackSplitQuantity,
             _bindings.Assets.DefaultFont,
             _bindings.Assets.DebugFont,
             _bindings.Assets.ResolveSprite,