fix: protocol method calls arity-check (issue 0131)

emitProtocolDispatch now requires the user-arg count to equal the
protocol method's parameter list — exact, since protocol signatures
have no defaults, packs, or variadics — and emits the same
"expects N arguments, but M were given" diagnostic plain calls get.
Previously extra args were silently dropped (and missing args left the
thunk reading garbage). The dispatch gains the call-site span for the
diagnostic. examples/1634 pins the rejection; full sweep confirms no
existing code relied on the leniency.

src/ir/lower/call.zig

@@ -861,7 +861,7 @@ pub fn lowerCall(self: *Lowering, c_in: *const ast.Call) Ref {
             // protocol value as its first param).
             if (self.getProtocolInfo(obj_ty)) |proto_info| {
                 if (protocolHasMethod(proto_info, fa.field)) {
-                    return self.emitProtocolDispatch(obj, proto_info, fa.field, args.items, obj_ty);
+                    return self.emitProtocolDispatch(obj, proto_info, fa.field, args.items, obj_ty, c.callee.span);
                 }
             }
 
@@ -878,7 +878,7 @@ pub fn lowerCall(self: *Lowering, c_in: *const ast.Call) Ref {
                     const pay_ty = opt_info.optional.child;
                     if (self.getProtocolInfo(pay_ty)) |proto_info| {
                         if (protocolHasMethod(proto_info, fa.field)) {
-                            return self.emitProtocolDispatch(obj, proto_info, fa.field, args.items, pay_ty);
+                            return self.emitProtocolDispatch(obj, proto_info, fa.field, args.items, pay_ty, c.callee.span);
                         }
                     }
                 }

src/ir/lower/protocol.zig

@@ -498,7 +498,7 @@ pub fn buildProtocolValue(self: *Lowering, concrete_ptr: Ref, proto_name: []cons
 
 /// Emit protocol method dispatch for a protocol-typed receiver.
 /// Returns the call result ref.
-pub fn emitProtocolDispatch(self: *Lowering, receiver: Ref, proto_info: ProtocolDeclInfo, method_name: []const u8, args: []const Ref, proto_ty: TypeId) Ref {
+pub fn emitProtocolDispatch(self: *Lowering, receiver: Ref, proto_info: ProtocolDeclInfo, method_name: []const u8, args: []const Ref, proto_ty: TypeId, span: ast.Span) Ref {
     // Find method index
     var method_idx: ?usize = null;
     var method_info: ?ProtocolMethodInfo = null;
@@ -512,6 +512,19 @@ pub fn emitProtocolDispatch(self: *Lowering, receiver: Ref, proto_info: Protocol
     const mi = method_info orelse return self.emitError(method_name, null);
     const midx = method_idx orelse 0;
 
+    // Arity is exact: a protocol signature has no defaults, packs, or
+    // variadics, so the user-arg count must equal its parameter list
+    // (issue 0131: extra args were silently dropped here; missing args
+    // left the thunk reading garbage).
+    if (args.len != mi.param_types.len) {
+        if (self.diagnostics) |d| {
+            const s: []const u8 = if (mi.param_types.len == 1) "" else "s";
+            const got_verb: []const u8 = if (args.len == 1) "was" else "were";
+            d.addFmt(.err, span, "'{s}' expects {d} argument{s}, but {d} {s} given", .{ method_name, mi.param_types.len, s, args.len, got_verb });
+        }
+        return Ref.none;
+    }
+
     // Extract ctx from protocol struct (field 0)
     const void_ptr = self.module.types.ptrTo(.void);
     const ctx = self.builder.structGet(receiver, 0, void_ptr);