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/Layout/SelectedObjectController.cs

@@ -93,6 +93,7 @@ public sealed class SelectedObjectController : IRetainedPanelController
     private readonly Action<uint>        _sendQueryItemMana;
     private readonly StackSplitQuantityState _splitQuantity;
     private readonly SelectionState _selection;
+    private readonly Func<uint, bool> _isVendorSplitExempt;
     private readonly Action<Action<uint, float>> _unsubscribeHealthChanged;
     private readonly Action<Action<uint, float, bool>> _unsubscribeItemManaChanged;
     private readonly Action<Action<ClientObject>> _unsubscribeObjectUpdated;
@@ -126,7 +127,8 @@ public sealed class SelectedObjectController : IRetainedPanelController
         UiDatFont?          datFont,
         StackSplitQuantityState splitQuantity,
         Action<Action<ClientObject>> subscribeObjectUpdated,
-        Action<Action<ClientObject>> unsubscribeObjectUpdated)
+        Action<Action<ClientObject>> unsubscribeObjectUpdated,
+        Func<uint, bool>    isVendorSplitExempt)
     {
         _isHealthTarget  = isHealthTarget;
         _isOwnedByPlayer = isOwnedByPlayer;
@@ -139,6 +141,8 @@ public sealed class SelectedObjectController : IRetainedPanelController
         _sendQueryItemMana = sendQueryItemMana;
         _splitQuantity = splitQuantity ?? throw new ArgumentNullException(nameof(splitQuantity));
         _selection = selection ?? throw new ArgumentNullException(nameof(selection));
+        _isVendorSplitExempt = isVendorSplitExempt
+            ?? throw new ArgumentNullException(nameof(isVendorSplitExempt));
         _unsubscribeHealthChanged = unsubscribeHealthChanged;
         _unsubscribeItemManaChanged = unsubscribeItemManaChanged;
         _unsubscribeObjectUpdated = unsubscribeObjectUpdated;
@@ -250,6 +254,18 @@ public sealed class SelectedObjectController : IRetainedPanelController
     /// <param name="stackSize">Returns the stack size for a guid (0 or 1 = non-stacked).</param>
     /// <param name="sendQueryHealth">Sends retail <c>QueryHealth (0x01BF)</c>; may be a no-op offline.</param>
     /// <param name="datFont">Dat font for the name label; null = debug bitmap font fallback.</param>
+    /// <param name="isVendorSplitExempt">
+    /// Slice 6.2: retail's <c>gmToolbarUI::HandleSelectionChanged</c> vendor
+    /// branch (<c>pc:198779-198790</c>) — true when the selected guid is
+    /// owned by the currently-open vendor (its <c>ClientObject.ContainerId</c>
+    /// equals <c>VendorState.VendorId</c>) AND its type intersects
+    /// <see cref="VendorSplitPolicy.SplitExemptMask"/>. When true, a stack
+    /// seeds to quantity 1 instead of the full authored stack size — see
+    /// <see cref="ApplySelection"/>. <c>VendorUiController.VendorSplitSize</c>
+    /// answers the SAME question for vendor's own display text via the SAME
+    /// <see cref="VendorSplitPolicy"/> helper, so the mask exists in exactly
+    /// one place (composed at <c>InteractionRetainedUiComposition</c>).
+    /// </param>
     public static SelectedObjectController Bind(
         ImportedLayout layout,
         SelectionState selection,
@@ -269,14 +285,16 @@ public sealed class SelectedObjectController : IRetainedPanelController
         UiDatFont?          datFont,
         StackSplitQuantityState splitQuantity,
         Action<Action<ClientObject>> subscribeObjectUpdated,
-        Action<Action<ClientObject>> unsubscribeObjectUpdated)
+        Action<Action<ClientObject>> unsubscribeObjectUpdated,
+        Func<uint, bool>    isVendorSplitExempt)
         => new SelectedObjectController(
             layout, selection,
             subscribeHealthChanged, unsubscribeHealthChanged,
             subscribeItemManaChanged, unsubscribeItemManaChanged,
             isHealthTarget, isOwnedByPlayer, name, healthPercent, hasHealth, stackSize,
             sendQueryHealth, manaPercent, sendQueryItemMana, datFont,
-            splitQuantity, subscribeObjectUpdated, unsubscribeObjectUpdated);
+            splitQuantity, subscribeObjectUpdated, unsubscribeObjectUpdated,
+            isVendorSplitExempt);
 
     /// <summary>
     /// Port of <c>gmToolbarUI::HandleSelectionChanged</c> (<c>:198635</c>):
@@ -335,11 +353,20 @@ public sealed class SelectedObjectController : IRetainedPanelController
 
         // gmToolbarUI::HandleSelectionChanged @ 0x004BF52D..0x004BF666:
         // stacks initialize to the full stack, show the numeric entry + horizontal
-        // slider, and set the stacked selection state. Vendor-owned stack precedence
-        // is intentionally absent until the vendor panel owns an active vendor id.
+        // slider, and set the stacked selection state. Slice 6.2: the
+        // vendor-owned branch (pc:198779-198790, mask literal pc:198784)
+        // seeds splitSize (the INITIAL value) to 1 instead of the full stack
+        // when the selection is owned by the currently-open vendor AND its
+        // type intersects VendorSplitPolicy.SplitExemptMask — see the
+        // isVendorSplitExempt parameter doc. maxSplitSize (the slider's
+        // RANGE) is always the full authored stack size regardless of
+        // exemption (research doc §B.3: "Sets GenItemHolder::splitSize =
+        // seed, GenItemHolder::maxSplitSize = stackSize") — only the
+        // starting VALUE differs, not the ceiling.
         if (stackSize > 1u)
         {
-            _splitQuantity.Reset(stackSize);
+            uint seed = _isVendorSplitExempt(g) ? 1u : stackSize;
+            _splitQuantity.Reset(stackSize, initialValue: seed);
             if (_stackSizeEntry is not null) _stackSizeEntry.Visible = true;
             if (_stackSizeSlider is not null) _stackSizeSlider.Visible = true;
         }