test: ww-native behavior harness (runww.ww) + 2 pilot cases

First piece of the Go-model test rebuild: runww.ww is a ww program (the
test/run.go analog) that drives compiler cases through BOTH stages and
asserts behavior/diagnostics. Directives //ww:run / //ww:run-exit N /
//ww:error "<substr>" / //ww:compile; spawn+stderr-capture lift the
driver's procrun (main.ww:160) + dup2(2). The //ww:error check requires
rc!=0 AND the diagnostic substring (the #20 non-vacuity guard -- a crash
can't pass), and a malformed error directive fails loudly.

Bulk corpus migration + ww test wiring are follow-up folds; case spawns
need os.envp() (#28) and per-pid /tmp paths (#29) first.

test/wcc/data/runww_enum_reject/case.ww

@@ -0,0 +1,8 @@
+//ww:error "enum storage type must be integer"
+// Lifted from test/wcc/850_enum_reject.c reject_row "nonint_stor": a
+// non-integer enum storage type must be LOUD-rejected by both stages.
+// The directive's substring is the shared diagnostic body (no file:line
+// prefix, which differs cstage vs wwstage) — the #20 non-vacuity anchor.
+package main;
+type e = enum f64 { A };
+fn main() i32 = { return e.A as i32; };

test/wcc/data/runww_enum_run/case.ww

@@ -0,0 +1,7 @@
+//ww:run-exit 6
+// Lifted from test/wcc/850_enum_reject.c ok_row "distinct": a valid enum
+// with forward member arithmetic must BUILD and run. main returns
+// e.A (0) + e.C (B+1 = 6) == 6, signalled via the process exit code.
+package main;
+type e = enum { A, B = 5, C = B + 1 };
+fn main() i32 = { return e.A as i32 + e.C as i32; };