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.

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;
+};