fix(vendor): re-gate residuals — MaxStackSize is the stack operand, wire-authored use radius, purse summaries

R1 the split bar's operand is the item's authored MaxStackSize —
three retail sites read pwd._maxStackSize directly (InqListSlotCount
pc:200052, buy-button cases pc:203996/204086) where ACE never fills
the desc stack and standard stock is unlimited. Threaded StackSizeMax
end to end with one shared resolver; the two literal _maxStackSize
sites are now byte-exact; AP-165 retired, AP-169 corrected.
R2 walk-to-vendor never opened because GetUseRadius used an UNCITED
3m Creature heuristic as the local stop distance while ACE's poll
demands the authored radius (default 0.6 m) — the walk stopped and
the Use fired far outside acceptance. Now reads the wire-authored
spawn UseRadius with ACE's exact fallback; heuristic constants
deleted. A first sabotage attempt was non-discriminating
(coincidental 0.6) and was corrected — the discriminating version is
what landed.
R3 the Buying/Selling purse summaries ("Buying %d %s worth %hsp" /
"You have %hsp") recovered from the binary data segment where BN
mis-attributes the Buy-side literal; wired to staging and money
changes on the four authored text elements; AP-166 narrowed to the
pending-sell highlight.

Clean-room complete solution: 11,528 passed / 4 skipped / 0 failed.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/AcDream.Core/Items/VendorSplitPolicy.cs

@@ -42,4 +42,37 @@ public static class VendorSplitPolicy
         IsSplitExempt(itemType)
             ? 1
             : authoredStackSize is { } size && size > 0 ? size : 1;
+
+    /// <summary>
+    /// Grand-gate finding R1 (2026-08-08, register AP-169 correction): the
+    /// retail-faithful "how big is one stack of this item" answer for a
+    /// VENDOR-owned selection, used everywhere retail reads
+    /// <c>PublicWeenieDesc::_stackSize</c> for splitting/seeding purposes
+    /// (<c>gmToolbarUI::HandleSelectionChanged</c>,
+    /// <c>pc:198688</c>/<c>198744</c>/<c>198774</c>/<c>198791</c>;
+    /// <c>ItemHolder::GetObjectSplitSize</c>, <c>pc:401465-401477</c>).
+    ///
+    /// <para>
+    /// Prefers <paramref name="descStackSize"/> (the wire's own
+    /// <c>PublicWeenieDesc::_stackSize</c> — retail-faithful FIRST, honored
+    /// unchanged if a real retail server or a future ACE fix ever populates
+    /// it). ACE never sets it for a vendor browse-list row
+    /// (<c>Vendor.LoadInventoryItem</c> never calls <c>wo.SetStackSize</c>),
+    /// so this falls back to <paramref name="maxStackSize"/> — the item
+    /// TYPE's authored stack ceiling, which ACE DOES populate (an ordinary
+    /// weenie property, not an instance-specific stack count) and which a
+    /// real retail server evidently uses AS <c>_stackSize</c> for an
+    /// unlimited-supply "one full stack" browse listing (the live retail
+    /// screenshot: a Prismatic Taper listing shows "1000 Prismatic Tapers",
+    /// 1000 being the taper's authored max stack size, not any bounded
+    /// supply count). Finally falls back to 1 (non-splittable) when neither
+    /// field is available. See the register, AP-169.
+    /// </para>
+    /// </summary>
+    public static int ResolveAuthoredStackSize(int? descStackSize, int? maxStackSize) =>
+        descStackSize is { } desc && desc > 0
+            ? desc
+            : maxStackSize is { } max && max > 0
+                ? max
+                : 1;
 }

src/AcDream.Core/Items/VendorState.cs

@@ -58,6 +58,23 @@ public readonly record struct VendorShopItem(
     // matching retail's own zeroed-struct default of 0 for the same case
     // (see VendorPricing.PerUnitValue's <= 0 guard).
     int? DescStackSize = null,
+    // Grand-gate finding R1 (2026-08-08, register AP-169 correction): the
+    // item TYPE's authored stack ceiling (retail PublicWeenieDesc::
+    // _maxStackSize, wire AcDream.Core.Net.Messages.PublicWeenieDescBody.
+    // StackSizeMax). Retail's OWN InqListSlotCount (pc:200038-200065) and
+    // the Buying tab's Buy Item/Clear Item removal rule
+    // (gmVendorUI::HandleButtonClicks cases 0x100000c9/0x100000cb,
+    // pc:203989-204010/204080-204094) read THIS field literally, never
+    // DescStackSize, to decide whether an item is stackable at all. It also
+    // serves as this port's retail-faithful SUBSTITUTE for the toolbar's
+    // splitSize/maxSplitSize seed (gmToolbarUI::HandleSelectionChanged,
+    // pc:198688/198744/198774/198791 — reads PublicWeenieDesc::_stackSize,
+    // which ACE never populates for a vendor browse-list row; a real retail
+    // server evidently authors _stackSize == _maxStackSize for an
+    // unlimited-supply "one full stack" browse listing, so MaxStackSize is
+    // the value retail's own server would have put there) — see
+    // VendorSplitPolicy.ResolveAuthoredStackSize and the register (AP-169).
+    int? MaxStackSize = null,
     // Review finding F5 (Slice 5.4 review): PublicWeenieDescBody already
     // carries these three (IconOverlayId/IconUnderlayId/UiEffects) — see
     // AcDream.Core.Net.Messages.PublicWeenieDescBody. Mirrors