flow: bounded iteration combinators flow-while/flow-until + 6 tests

(flow-while pred body max) / (flow-until pred body max) re-run body threading the
value while/until pred holds, capped at max steps for a deterministic bound (no
unbounded loops in pure SX). 122/122 across 7 suites.

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

plans/flow-on-sx.md

@@ -16,7 +16,7 @@ federation extension via fed-sx for remote-node execution.
 
 ## Status (rolling)
 
-`bash lib/flow/conformance.sh` → **116/116** (Phases 1-5 complete)
+`bash lib/flow/conformance.sh` → **122/122** (Phases 1-5 complete)
 
 ## Ground rules
 
@@ -145,7 +145,9 @@ something operators and authors actually use. Accumulation, not a rewrite.
 - [x] `recover` — complement to try-catch for the fail-VALUE channel: run node; if it
   yields `(fail ...)`, run a recovery node on the reason
 - [x] `map-flow` — run a flow per item of a list, join results (sequential)
-- [x] `lib/flow/tests/api.sx` (12) + `lib/flow/tests/combinators.sx` (11)
+- [x] `flow-while` / `flow-until` — bounded iteration: re-run body threading the
+  value while/until pred holds, capped at `max` steps (deterministic bound)
+- [x] `lib/flow/tests/api.sx` (12) + `lib/flow/tests/combinators.sx` (17)
 
 ## Progress log