wcc/ww: reject (a,) single-element trailing-comma tuple (catB-92)

wwstage's tuple-parse loop checked the RPAREN-break at the top, so
`(a,)` parsed as a 1-element N_TUPLE and reached cgen — a silent
wrong-accept. A trailing comma is legal only after >=2 elements.
Align the loop order to cstage cmd/wcc/parse.c:552-558 (parse each
element before the RPAREN-break); `(a,)` now errors at the next
parseexpr, `(a, b)` / `(a, b,)` are unchanged. cstage already
rejected; this brings wwstage's w6c_ww parser into agreement.

Regen w6c/wwdump combined.ww (parser embeds in both). Valid-program
codegen unchanged → cs==ww byte-id gate stays green.

selfhost/cmd/wwdump/main.combined.ww

@@ -8268,12 +8268,16 @@ fn parseprimary(p: *parser) *node = {
 			let t: *node = newnode(nkind.N_TUPLE, pf, pl, pc);
 			t.list = e;
 			let tail: *node = e;
+			// Parse each element BEFORE the RPAREN-break so `(a,)`
+			// (a single elem + trailing comma) is a loud parse error;
+			// a trailing comma is legal only after >=2 elems. Mirror
+			// cstage cmd/wcc/parse.c:552-558 loop order.
 			for (true) {
-				if (p.curkind == tkind.TK_RPAREN) { break; };
 				let en: *node = parseexpr(p);
 				tail.next = en;
 				tail = en;
 				if (!accepttok(p, tkind.TK_COMMA)) { break; };
+				if (p.curkind == tkind.TK_RPAREN) { break; };
 			};
 			expecttok(p, tkind.TK_RPAREN, "expected ')' in tuple");
 			return t;