wcc: wwstage checks call arity at desugarcallargs

cstage rejects `add(1,2,3)` and `add(4)` (too many / not enough
arguments); wwstage ran no count check at this seam, so both built
-- a stray arg pushed silently, a missing one read garbage
(cs-reject/ww-accept build-verdict divergence). Surplus errs when
params exhaust with args left; missing errs on a leftover regular
param (a leftover TK_ELLIPSIS or FFI "..." is a legal zero-arg
variadic tail). fn-VALUE callees still bail at decl==nil -- their
whole typecheck, arity included, stays task #51.

internal/wwfixture/types.ww

@@ -1,13 +1,13 @@
 package wwfixture;
 
 def protocolversion: i32 = 1;
-def corpuscount: i32 = 1752;
+def corpuscount: i32 = 1754;
-def errorcount: i32 = 349;
+def errorcount: i32 = 351;
 def compilecount: i32 = 21;
 def runcount: i32 = 209;
 def runexitcount: i32 = 1173;
-def nativecount: i32 = 3504;
+def nativecount: i32 = 3508;
-def corpushash: str = "d3c7b6a94918a9c9af14df029ef0cdf6d976a3f1351d41ff9db9a69b753cb016";
+def corpushash: str = "67a119b41fd78b1f2900c096e5b5670e98935d2965573f67bb21ab817931dffa";
 
 type directive = enum i32 {
 	ERROR = 0,

selfhost/cmd/wcc/check.ww

@@ -6063,6 +6063,19 @@ fn desugarcallargs(c: *checker, n: *syntax.node) void = {
 	let a: *syntax.node = n.list;
 	for (a != nil) {
 		let nexta: *syntax.node = a.next;
+		// Arity, surplus side — params exhausted with args left
+		// (variadics never exhaust: TK_ELLIPSIS absorbs without
+		// advancing, the FFI "..." breaks out below). cstage errs
+		// at check.c N_CALL `too many arguments`; wwstage ran no
+		// count check at all, so `add(1,2,3)` built and pushed a
+		// stray arg silently (cs-reject/ww-accept divergence).
+		// fn-VALUE callees bail above decl==nil — task #51 owns
+		// their whole typecheck, arity included.
+		if (param == nil) {
+			cerr("error: too many arguments\n");
+			c.errs += 1;
+			return;
+		};
 		if (param != nil) {
 			if (param.kind == syntax.nkind.N_PARAM) {
 				// C-style FFI variadic (bare `...`): str=="...",
@@ -6134,6 +6147,16 @@ fn desugarcallargs(c: *checker, n: *syntax.node) void = {
 		prev = a;
 		a = nexta;
 	};
+	// Arity, missing side — a regular param left over after the
+	// args ran out (a leftover TK_ELLIPSIS or FFI "..." is a legal
+	// zero-arg variadic tail). Mirror cstage `not enough arguments`.
+	if (param != nil) { if (param.kind == syntax.nkind.N_PARAM) {
+		if (param.op != syntax.tkind.TK_ELLIPSIS
+		    && !syntax.streq(param.str, "...")) {
+			cerr("error: not enough arguments\n");
+			c.errs += 1;
+		};
+	}; };
 };
 
 // checkassign — #258 at the assignment context. wwstage runs no other

test/wcc/data/call_arity_missing/case.ww

@@ -0,0 +1,8 @@
+//ww:error "not enough arguments"
+// Arity, missing side: the callee read a garbage second param slot.
+package main;
+fn add(a: i64, b: i64) i64 = { return a + b; };
+export fn main() i32 = {
+	let x: i64 = add(4);
+	return x: i32;
+};

test/wcc/data/call_arity_surplus/case.ww

@@ -0,0 +1,9 @@
+//ww:error "too many arguments"
+// Arity, surplus side: wwstage desugarcallargs ran no count check, so
+// a stray arg built and pushed silently (cs-reject/ww-accept).
+package main;
+fn add(a: i64, b: i64) i64 = { return a + b; };
+export fn main() i32 = {
+	let x: i64 = add(1, 2, 3);
+	return x: i32;
+};