fix(vendor): 6b/6c review corrections — pre-send guards, accumulating staging, trade-note exemption, drag-over tab switch, full-stack sells

All thirteen findings, each anchored in recovered bytes or pc reads:

Buy All now runs retail's four PRE-SEND guards in order (pyreal and
alt-currency affordability, container and item slot capacity; strings
recovered from .rdata at 0x007b57b4/0x007b5750) — a rejected batch can
no longer destroy the staged list. Staged adds ACCUMULATE with the
5000 cap ("I can't possibly sell you that much!..." @0x007b59d8) and
the shop rows decrement/restore per RemoveFromShop. The max-value sell
rejection exempts trade notes — the raw bytes at 0x005d1add are `not`
(bitwise), not the pseudo-C's misleading `!`, and the early ret skips
the min check too. BF_RETAINED gates selling end to end (the bit was
already on ClientObject; AP-164's three claims were all false once
traced — RETIRED). Dragging over the vendor window auto-opens the
Selling tab per UpdateDragOver — with a correction to the review's own
citation: token 0x100000cd is the SELLING page, the guard is
"don't reopen the current tab." Sells are full-stack-only (three
retail sites; "Cannot sell part of a stack" @0x007b57ec) and Sell Item
acts on the global selection unconditionally. The confirm string gains
its byte-true trailing '?', dies with the session, staged-row
highlights repaint, dead guids unstage with retail's shopping-list
notice, and move-to-use no longer walks to targets the dispatch would
refuse.

AP-162 narrowed, AP-164 retired, AP-167/AP-168 filed honest.

Clean-room complete solution: 11,508 passed / 4 skipped / 0 failed.

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

src/AcDream.Core/Items/ItemInteractionPolicy.cs

@@ -26,6 +26,13 @@ public enum PublicWeenieFlags : uint
     Healer = 0x00010000,
     Lockpick = 0x00020000,
     RequiresPackSlot = 0x00800000,
+    /// <summary>
+    /// F4 (Slice 6b/6c review): <c>BF_RETAINED</c>, the "unsellable" bit
+    /// <c>VendorProfile::InqAcceptability</c> tests (<c>pc:005d1aa7</c>,
+    /// byte 3 bit 0 of <c>PublicWeenieDesc::_bitfield</c>). See
+    /// <see cref="VendorSellAcceptability"/>.
+    /// </summary>
+    Retained = 0x01000000,
     VolatileRare = 0x10000000,
     WieldOnUse = 0x20000000,
     WieldLeft = 0x40000000,