selfhost: add lib/Drop.bux with interface Drop; extend methods visible in scope

- lib/Drop.bux: interface Drop { func Drop(self: *Self); }
- sema.bux: define TypeName_MethodName in scope for impl block methods
  so auto-drop (Lcx_BuildAutoDropFree) can find Drop implementations
  via extend Type for Drop, not just @[Drop] or standalone functions
- Verified: extend Buffer for Drop auto-calls Buffer_Drop in @[Checked]

lib/Drop.bux

@@ -0,0 +1,4 @@
+// Drop trait — automatic cleanup for heap-allocated or resource-holding types
+interface Drop {
+    func Drop(self: *Self);
+}

src/sema.bux

@@ -1148,12 +1148,25 @@ func Sema_CollectGlobals(sema: *Sema) {
             let implTypeName: String = decl.strValue;
             var m: *Decl = decl.childDecl1;
             while m != null as *Decl {
-                if m.kind == dkFunc && sema.methodCount < 256 {
+                if m.kind == dkFunc {
+                    if sema.methodCount < 256 {
                         sema.methodEntries[sema.methodCount].typeName = implTypeName;
                         sema.methodEntries[sema.methodCount].methodName = m.strValue;
                         sema.methodEntries[sema.methodCount].decl = m;
                         sema.methodCount = sema.methodCount + 1;
                     }
+                    // Also define TypeName_MethodName in scope so auto-drop can find it
+                    let methodSymName: String = String_Concat(implTypeName, "_");
+                    let methodSymName2: String = String_Concat(methodSymName, m.strValue);
+                    var methodSym: Symbol;
+                    Sema_ZeroInitSymbol(&methodSym);
+                    methodSym.kind = skFunc;
+                    methodSym.name = methodSymName2;
+                    methodSym.typeKind = tyFunc;
+                    methodSym.isPublic = decl.isPublic;
+                    methodSym.decl = m;
+                    discard Scope_Define(sema.scope, methodSym);
+                }
                 m = m.childDecl2;
             }
         }