erlang: lists foldr/3 + partition/2 + takewhile/dropwhile/splitwith (823/823)

Higher-order traversal family in lib/erlang/lists-ext.sx, registered
pure via the er-register-builtin-bifs! wrapper. foldr right-folds;
partition returns {Yes,No} order-preserved; splitwith = {takewhile,
dropwhile}. lists_ext suite 38 -> 52.

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

lib/erlang/scoreboard.md

@@ -1,6 +1,6 @@
 # Erlang-on-SX Scoreboard
 
-**Total: 809 / 809 tests passing**
+**Total: 823 / 823 tests passing**
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
@@ -16,7 +16,7 @@
 | ✅ | ffi | 37 | 37 |
 | ✅ | vm | 78 | 78 |
 | ✅ | send_after | 10 | 10 |
-| ✅ | lists_ext | 38 | 38 |
+| ✅ | lists_ext | 52 | 52 |
 
 
 Generated by `lib/erlang/conformance.sh`.