flow: combinator library — tap, recover, map-flow + 11 tests (Phase 5 complete)

tap: side-effecting pass-through (returns input). recover: fail-VALUE counterpart
of try-catch (run node; on (fail r) run handler on r). map-flow: run a node over
each item of a list, join results sequentially. 116/116 across 7 suites.

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

lib/flow/conformance.sh

@@ -27,6 +27,7 @@ SUITES=(
   "recovery    flow-rec-tests-run!       lib/flow/tests/recovery.sx"
   "distributed flow-dist-tests-run!      lib/flow/tests/distributed.sx"
   "api         flow-api-tests-run!       lib/flow/tests/api.sx"
+  "combinators flow-cmb-tests-run!       lib/flow/tests/combinators.sx"
 )
 
 TMPFILE=$(mktemp); trap "rm -f $TMPFILE" EXIT