phase: 20.49 COMPLETE; 20.50 Flow+String minimal reps; 20.51 selfhost v0/v1 minimal (Option A/B); hv1-inline binop/unop/copy; docs + run_all + CURRENT_TASK -> 21.0

tools/dev_env.sh

@@ -105,6 +105,8 @@ activate_prod() {
   # Disallow AST prelude merge and file using
   export NYASH_USING_AST=0
   export NYASH_ALLOW_USING_FILE=0
+  # Mark using profile as prod to enable strict path-using errors
+  export NYASH_USING_PROFILE=prod
   # Keep using gate explicit; alias resolution allowed via resolver (no AST merge)
   : "${NYASH_ENABLE_USING:=1}"
   # Quiet resolver by default