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/Composition/InteractionRetainedUiComposition.cs

@@ -361,7 +361,16 @@ internal sealed class RetailInteractionRetainedUiCompositionFactory
             {
                 d.Inventory.ExternalContainers.RequestOpen(guid);
             },
-            requestUse: selection.RequestUse);
+            requestUse: selection.RequestUse,
+            // Slice 6.3: ItemInteractionController.TryBuy owns the
+            // reservation dance itself (see its doc comment); this is a
+            // plain wire send, not a second requestUse-shaped delegate.
+            sendBuy: (vendorGuid, itemGuid, amount, alternateCurrencyId) =>
+                session.CurrentSession?.SendBuy(
+                    vendorGuid,
+                    itemGuid,
+                    amount,
+                    alternateCurrencyId));
     }
 
     public MagicRuntime CreateMagicRuntime(
@@ -655,7 +664,21 @@ internal sealed class RetailInteractionRetainedUiCompositionFactory
                         late.Session.CurrentSession?.SendPutItemInContainer(
                             item,
                             container,
-                            placement)),
+                            placement),
+                    // Slice 6.2: retail gmToolbarUI::HandleSelectionChanged's
+                    // own vendor-owned gate (pc:198781) is literally
+                    // "selected weenie's ContainerId == the open vendor's
+                    // guid" — Slice 6.1 guarantees a materialized shop
+                    // item's ContainerId IS the vendor's guid, so this reads
+                    // straight off the same ClientObjectTable/VendorState
+                    // pair VendorUiController.VendorSplitSize's display-only
+                    // copy also reads, through the SAME VendorSplitPolicy
+                    // mask helper (no second mask copy).
+                    guid =>
+                        d.Inventory.Vendor.VendorId != 0u
+                        && d.Inventory.Objects.Get(guid) is { } vendorCandidate
+                        && vendorCandidate.ContainerId == d.Inventory.Vendor.VendorId
+                        && VendorSplitPolicy.IsSplitExempt(vendorCandidate.Type)),
                 Character: new CharacterRuntimeBindings(characterSheet),
                 Inventory: new InventoryRuntimeBindings(
                     d.Inventory.Objects,
@@ -706,7 +729,9 @@ internal sealed class RetailInteractionRetainedUiCompositionFactory
                     late.Selection.IsWithinExternalContainerUseRange),
                 Vendor: new VendorRuntimeBindings(
                     d.Inventory.Vendor,
-                    iconComposer.GetIcon),
+                    iconComposer.GetIcon,
+                    itemInteraction,
+                    d.Actions.Selection),
                 Cursor: new RetailUiCursorBindings(cursorFeedback, cursorManager),
                 Confirmations: new ConfirmationRuntimeBindings(
                     (type, context, accepted) =>