test: 995_self_rebuild — wwstage rebuilds itself byte-identical

Drives ww_ww (which already shells to 6c_ww/6a_ww/6l_ww) over each
wwstage tool's source and diffs the resulting binary against the
cstage-built canonical in $BIN. A green run means the toolchain
can recompile itself end-to-end without invoking cc, modulo the
cold-start binary that brings the wwstage into existence.

Stricter than `make bootstrap`: that loop pins wwdump's cgen
self-stabilising; this pins all five wwstage tools (6c, 6a, 6l,
ww, wwdump) round-tripping through the wwstage pipeline.

The .combined.ww refreshes are the expander picking up the
parser/cgen changes from the prior commit. selfhost/cmd/6c/
gains its main.combined.ww for the first time — 995 builds it,
994 reads it.

selfhost/cmd/6a/main.combined.ww

@@ -24,6 +24,7 @@ def SYS_OPEN:   i64 = 2;
 def SYS_CLOSE:  i64 = 3;
 def SYS_LSEEK:  i64 = 8;
 def SYS_ACCESS: i64 = 21;
+def SYS_DUP2:   i64 = 33;
 def SYS_GETPID: i64 = 39;
 def SYS_FORK:   i64 = 57;
 def SYS_EXECVE: i64 = 59;
@@ -62,6 +63,14 @@ export fn close(fd: i32) i32 = {
 	return syscall1(SYS_CLOSE, fd: i64): i32;
 };
 
+// dup2(2): make `newfd` refer to the same description as `oldfd`,
+// closing `newfd` first if open. Returns `newfd` on success or a
+// negative errno. Used by 6c_ww to redirect stdout into an output
+// file without changing the cgen emit path.
+export fn dup2(oldfd: i32, newfd: i32) i32 = {
+	return syscall2(SYS_DUP2, oldfd: i64, newfd: i64): i32;
+};
+
 // Fallible wrappers. The error variant is a plain str (Plan 9 errstr
 // model, see lib/errors); the sum type makes success/failure explicit
 // without overloading length-zero.