erlang: lists flatmap/2 + filtermap/2 + mapfoldl/3 + search/2 (862/862)

Higher-order list ops in lib/erlang/lists-ext.sx. filtermap honours
true/false/{true,V}; mapfoldl returns {Mapped, Acc}; search returns
{value,E}|false. lists_ext suite 83 -> 91.

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

plans/erlang-on-sx.md

@@ -159,6 +159,8 @@ The Phase 9 opcodes are registered, tested, and bridged SX↔OCaml, but inert: n
 
 _Newest first._
 
+- **2026-06-30 stdlib hardening — `lists` flatmap/filtermap/mapfoldl/search** — Added `flatmap/2`, `filtermap/2` (`true` keep / `false` drop / `{true, V}` transform), `mapfoldl/3` (returns `{MappedList, AccFinal}`), `search/2` (`{value, E}` | `false`) to `lib/erlang/lists-ext.sx`. `lists_ext` suite 83→**91** (+8). Conformance **854 → 862/862**. The `lists` module is now broadly covered (sort/usort/keylists/fold/partition/while/flatten/min/max/zip/slicing/flatmap/filtermap/mapfoldl/search on top of the originals). loops/erlang only.
+
 - **2026-06-30 stdlib hardening — `lists` slicing** — Added `sublist/2`, `sublist/3`, `nthtail/2`, `split/2`, `droplast/1` to `lib/erlang/lists-ext.sx`. `sublist` is lenient (clamps to list length); `nthtail/2` and `split/2` are strict (`badarg` when the list is shorter than N, matching the stdlib); `droplast/1` raises on `[]`. `lists_ext` suite 70→**83** (+13). Conformance **841 → 854/854**. loops/erlang only.
 
 - **2026-06-30 stdlib hardening — `lists` zip family** — Added `zip/2`, `zipwith/3`, `unzip/1` to `lib/erlang/lists-ext.sx`. Length mismatch (zip/zipwith) and malformed/non-pair elements (unzip) raise `badarg` (port equivalent of Erlang's `function_clause`). `lists_ext` suite 62→**70** (+8, incl. a zip/unzip roundtrip). Conformance **833 → 841/841**. loops/erlang only.