syntax: accept the empty for () condition (align up to cstage)

cstage parse.c:1027 accepts `for ()` — N_FOR with a nil cond, no
for-else arm, no test emitted (an unconditional loop; a deliberate
Go-flavored ww divergence from Hare, original to the toolchain
import). The wwstage parser had no RPAREN early-out and rejected it.
Selfhost checker and cgen already nil-guard n.cond, so the parser arm
is the whole gap. Graduates the held e2e row (pin 1731/1157/3462).

Adjacent asymmetries left filed, not fixed here (rule 11): the brace
form `for { }`, the empty middle clause of the 3-clause form, and
the REVERSE case (wwstage accepts 2-clause `for (cond; post)` which
cstage rejects).

test/wcc/data/r700_break_loop/case.ww

@@ -0,0 +1,11 @@
+//ww:run-exit 7
+// Migrated from 700_e2e row 23.
+package main;
+fn main() i32 = {
+  let i: i32 = 0;
+  for () {
+    i += 1;
+    if (i == 7) { break; };
+  };
+  return i;
+};