Phase 7: PlayerVendorSale core event + subscriber

The one non-drop-in piece. Player-vendor purchases raise no EventSink, so the
sale is invisible to subscription. Two git-format core patches add a
PlayerVendorSale event and raise it at the committed sale in
PlayerVendorBuyGump.OnResponse (right after HoldGold +=), where buyer, vendor
owner, item, price, and commission are all in scope. The subscriber
BridgeVendorSale emits vendor.sale.

All three are a coupled unit. The subscriber references PlayerVendorSaleEventArgs,
which does not exist until the EventSink patch is applied, so it lives in patches/
not overlay/ -- shipping it in overlay would break the build on any unpatched
install. patches/README.md documents applying the unit; both patches verified
with git apply --check against stock ServUO 57.4. This is the first phase that
rebuilds the core (ServUO.exe), not just Scripts.dll.

vendor.sale carries buyer and vendor-owner accounts, both present and distinct,
which is the pair that flags gold-laundering when they match -- richer than the
ownerless NPC ValidVendor* events, and on a committed sale rather than a
validation stage.

Verified with a probe firing the event on real seeded-vendor data: vendor.sale
emitted with buyerAcct=seed_001, ownerAcct=seed_000, Longsword, price 69819. The
probe proves the event, args, subscriber, and payload; the literal gump call site
firing on a real purchase needs a live buyer with a NetState and is confirmed by
an in-game buy. Evidence in docs/PLAN.md §17.

This completes every phase on the ServUO side. Phases 0-6 are drop-in (overlay/);
7 is patches/. Cheat signals are folded into existing streams (fastwalk, audit,
vendor.sale), not a separate phase. Remaining work is the Rust sidecar.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

patches/playervendor-sale-gump.patch

@@ -0,0 +1,15 @@
+diff --git a/Scripts/Gumps/PlayerVendorGumps.cs b/Scripts/Gumps/PlayerVendorGumps.cs
+index 049aae6..f1b30d2 100644
+--- a/Scripts/Gumps/PlayerVendorGumps.cs
++++ b/Scripts/Gumps/PlayerVendorGumps.cs
+@@ -95,6 +95,10 @@ namespace Server.Gumps
+ 
+                     m_Vendor.HoldGold += m_VI.Price - commission;
+ 
++                    // uo-link: the only committed-sale hook for player vendors (no EventSink exists).
++                    EventSink.InvokePlayerVendorSale(
++                        new PlayerVendorSaleEventArgs(from, m_Vendor, m_Vendor.Owner, m_VI.Item, m_VI.Price, commission));
++
+                     from.SendLocalizedMessage(503201); // You take the item.
+                 }
+             }