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.
