syntax: align for-header grammar to cstage (Go's three forms)

The bbdd8bed residue, ruled by the Go derivation (ww's for IS the Go
for; the cstage grammar is the language definition and already the
Go-shaped one):
- bare `for { }` accepted (align UP to parse.c:1023, incl. for-else);
- 3-clause cond and post each omissible — `for (init;; post)`,
  `for (init; cond)`, `for (init;)` (align UP, parse.c:1092-1097);
- 2-clause `for (cond; post)` REJECTED (align DOWN): the form exists
  in neither Go nor cstage (parse.c:1101 eats a stray ';' then
  requires ')'), so the old wwstage accept was a parser bug. The
  reject wording mirrors cstage's failure mode per stage.

forhdr_{brace,emptymid,partial} run fixtures pin the accepted forms
byte-identically; forhdr_twoclause_reject pins both reject fragments.
Corpus pin 1745/346/21/209/1169/3490.

internal/wwfixture/types.ww

@@ -1,13 +1,13 @@
 package wwfixture;
 
 def protocolversion: i32 = 1;
-def corpuscount: i32 = 1741;
-def errorcount: i32 = 345;
+def corpuscount: i32 = 1745;
+def errorcount: i32 = 346;
 def compilecount: i32 = 21;
 def runcount: i32 = 209;
-def runexitcount: i32 = 1166;
-def nativecount: i32 = 3482;
-def corpushash: str = "bad5b37d555f3f4d742f7a6fcfa857ff46315757233819a19eb9bc1da2800ce7";
+def runexitcount: i32 = 1169;
+def nativecount: i32 = 3490;
+def corpushash: str = "472974a00552c6b3f5db1fc6b0c1ed128d7554e134880196c833f11f99370f21";
 
 type directive = enum i32 {
 	ERROR = 0,