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-eventsink.patch

@@ -0,0 +1,66 @@
+diff --git a/Server/EventSink.cs b/Server/EventSink.cs
+index d30788f..1da2667 100644
+--- a/Server/EventSink.cs
++++ b/Server/EventSink.cs
+@@ -171,6 +171,8 @@ namespace Server
+ 
+     public delegate void ValidVendorSellEventHandler(ValidVendorSellEventArgs e);
+ 
++    public delegate void PlayerVendorSaleEventHandler(PlayerVendorSaleEventArgs e);
++
+     public delegate void CorpseLootEventHandler(CorpseLootEventArgs e);
+ 
+     public delegate void RepairItemEventHandler(RepairItemEventArgs e);
+@@ -1521,6 +1523,29 @@ namespace Server
+         }
+     }
+ 
++    // Player-vendor purchases raise no other EventSink. This fires at the committed sale in
++    // PlayerVendorBuyGump.OnResponse, where buyer, vendor owner, item, price, and commission
++    // are all in scope -- the data the bridge's cheat-detection feed needs.
++    public class PlayerVendorSaleEventArgs : EventArgs
++    {
++        public Mobile Buyer { get; set; }
++        public Mobile Vendor { get; set; }
++        public Mobile Owner { get; set; }
++        public Item Item { get; set; }
++        public int Price { get; set; }
++        public int Commission { get; set; }
++
++        public PlayerVendorSaleEventArgs(Mobile buyer, Mobile vendor, Mobile owner, Item item, int price, int commission)
++        {
++            Buyer = buyer;
++            Vendor = vendor;
++            Owner = owner;
++            Item = item;
++            Price = price;
++            Commission = commission;
++        }
++    }
++
+     public class CorpseLootEventArgs : EventArgs
+     {
+         public Mobile Mobile { get; set; }
+@@ -1771,6 +1796,7 @@ namespace Server
+         public static event TameCreatureEventHandler TameCreature;
+         public static event ValidVendorPurchaseEventHandler ValidVendorPurchase;
+         public static event ValidVendorSellEventHandler ValidVendorSell;
++        public static event PlayerVendorSaleEventHandler PlayerVendorSale;
+         public static event CorpseLootEventHandler CorpseLoot;
+         public static event RepairItemEventHandler RepairItem;
+         public static event AlterItemEventHandler AlterItem;
+@@ -2416,6 +2442,14 @@ namespace Server
+             }
+         }
+ 
++        public static void InvokePlayerVendorSale(PlayerVendorSaleEventArgs e)
++        {
++            if (PlayerVendorSale != null)
++            {
++                PlayerVendorSale(e);
++            }
++        }
++
+         public static void InvokeCorpseLoot(CorpseLootEventArgs e)
+         {
+             if (CorpseLoot != null)