erlang: lists:sort/1,2 + lists:usort/1 with full term order (lists_ext suite, 788/788)

New lib/erlang/lists-ext.sx (loaded after runtime.sx): stable merge
sort over an SX-list bridge. sort/1 and usort/1 use full Erlang term
order via a self-contained er-ext-lt? (deep tuple/list compare that
the shared er-lt? lacks); sort/2 takes a fun(A,B)->bool comparator.

Registration wraps er-register-builtin-bifs! so the BIFs survive the
mid-run registry resets done by tests/runtime.sx.

Roadmap is saturated within this loop's scope; this is forever-loop
stdlib hardening. New file forced by the broken sx-tree write tools
in this worktree (see Blockers) — authored via Write + sx_validate.

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

lib/erlang/scoreboard.json

@@ -1,7 +1,7 @@
 {
   "language": "erlang",
-  "total_pass": 771,
-  "total": 771,
+  "total_pass": 788,
+  "total": 788,
   "suites": [
     {"name":"tokenize","pass":62,"total":62,"status":"ok"},
     {"name":"parse","pass":52,"total":52,"status":"ok"},
@@ -14,6 +14,7 @@
     {"name":"fib","pass":8,"total":8,"status":"ok"},
     {"name":"ffi","pass":37,"total":37,"status":"ok"},
     {"name":"vm","pass":78,"total":78,"status":"ok"},
-    {"name":"send_after","pass":10,"total":10,"status":"ok"}
+    {"name":"send_after","pass":10,"total":10,"status":"ok"},
+    {"name":"lists_ext","pass":17,"total":17,"status":"ok"}
   ]
 }