dream: static file serving — mime, etags, 304, ranges, traversal guard + 28 tests

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

lib/dream/conformance.sh

@@ -29,6 +29,7 @@ MODULES=(
   "lib/dream/flash.sx"
   "lib/dream/form.sx"
   "lib/dream/websocket.sx"
+  "lib/dream/static.sx"
 )
 
 # Suites: NAME RUNNER-FN PATH
@@ -40,6 +41,7 @@ SUITES=(
   "flash       dream-fl-tests-run!  lib/dream/tests/flash.sx"
   "form        dream-fo-tests-run!  lib/dream/tests/form.sx"
   "websocket   dream-ws-tests-run!  lib/dream/tests/websocket.sx"
+  "static      dream-st-tests-run!  lib/dream/tests/static.sx"
 )
 
 TMPFILE=$(mktemp); trap "rm -f $TMPFILE" EXIT