dream: flash messages — single-request cookie store + 14 tests

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/dream/conformance.sh

@@ -26,6 +26,7 @@ MODULES=(
   "lib/dream/router.sx"
   "lib/dream/middleware.sx"
   "lib/dream/session.sx"
+  "lib/dream/flash.sx"
 )
 
 # Suites: NAME RUNNER-FN PATH
@@ -34,6 +35,7 @@ SUITES=(
   "router      dream-rt-tests-run!  lib/dream/tests/router.sx"
   "middleware  dream-mw-tests-run!  lib/dream/tests/middleware.sx"
   "session     dream-ss-tests-run!  lib/dream/tests/session.sx"
+  "flash       dream-fl-tests-run!  lib/dream/tests/flash.sx"
 )
 
 TMPFILE=$(mktemp); trap "rm -f $TMPFILE" EXIT