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.Core/Items/VendorSplitPolicy.cs

@@ -0,0 +1,45 @@
+namespace AcDream.Core.Items;
+
+/// <summary>
+/// Retail's vendor-owned stack-split-seed rule — the SHARED mask check
+/// <c>gmToolbarUI::HandleSelectionChanged</c> applies when seeding
+/// <c>GenItemHolder::splitSize</c> for a vendor-owned selection
+/// (<c>pc:198779-198790</c>, mask literal at <c>pc:198784</c>): 1 unit if
+/// the item's type intersects the mask, else the item's own authored stack
+/// size.
+///
+/// <para>
+/// Single source of truth for the <c>0xDC41CB0</c> mask (Slice 6.2, per the
+/// research doc's §C.1 open question). Before this class existed the mask
+/// was ported once, privately, inside <c>VendorUiController</c>
+/// (display-only price/name text). <c>SelectedObjectController</c>'s real
+/// seeding path (the toolbar quantity slider — the actual retail mechanism
+/// this display text mirrors) needs the SAME check; both now call this
+/// class instead of either carrying its own copy.
+/// </para>
+/// </summary>
+public static class VendorSplitPolicy
+{
+    public const uint SplitExemptMask = 0x0DC41CB0u;
+
+    /// <summary>
+    /// True when <paramref name="itemType"/> intersects
+    /// <see cref="SplitExemptMask"/> — a vendor-owned selection of this type
+    /// always seeds/prices/names as quantity 1, regardless of its authored
+    /// stack size.
+    /// </summary>
+    public static bool IsSplitExempt(ItemType itemType) =>
+        ((uint)itemType & SplitExemptMask) != 0u;
+
+    /// <summary>
+    /// The quantity retail prices/names/seeds a VENDOR-OWNED selection at:
+    /// 1 if <paramref name="itemType"/> intersects <see cref="SplitExemptMask"/>,
+    /// else <paramref name="authoredStackSize"/> (0/absent treated as 1,
+    /// matching <c>ItemHolder::GetObjectSplitSize</c>'s own
+    /// <c>stackSize==0 -&gt; 1</c> floor, <c>pc:401473-401476</c>).
+    /// </summary>
+    public static int SeedQuantity(ItemType itemType, int? authoredStackSize) =>
+        IsSplitExempt(itemType)
+            ? 1
+            : authoredStackSize is { } size && size > 0 ? size : 1;
+}

src/AcDream.Core/Selection/SelectionState.cs

@@ -13,6 +13,11 @@ public enum SelectionChangeSource
     Toolbar,
     Keyboard,
     Plugin,
+    // Slice 6.2: a vendor shop-list row click — research doc §B.4 confirms
+    // vendor-context selections flow through the SAME global
+    // ACCWeenieObject::SetSelectedObject primitive as every other origin
+    // (VendorSellUI::AddItemToSell, pc:203558), never a vendor-local one.
+    Vendor,
 }
 
 public enum SelectionChangeReason