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/conformance.sh

@@ -39,6 +39,7 @@ SUITES=(
   "ffi|er-ffi-test-pass|er-ffi-test-count"
   "vm|er-vm-test-pass|er-vm-test-count"
   "send_after|er-sa-test-pass|er-sa-test-count"
+  "lists_ext|er-lx-test-pass|er-lx-test-count"
 )
 
 cat > "$TMPFILE" << 'EPOCHS'
@@ -50,6 +51,7 @@ cat > "$TMPFILE" << 'EPOCHS'
 (load "lib/erlang/parser-module.sx")
 (load "lib/erlang/transpile.sx")
 (load "lib/erlang/runtime.sx")
+(load "lib/erlang/lists-ext.sx")
 (load "lib/erlang/tests/tokenize.sx")
 (load "lib/erlang/tests/parse.sx")
 (load "lib/erlang/tests/eval.sx")
@@ -63,6 +65,7 @@ cat > "$TMPFILE" << 'EPOCHS'
 (load "lib/erlang/tests/ffi.sx")
 (load "lib/erlang/tests/vm.sx")
 (load "lib/erlang/tests/send_after.sx")
+(load "lib/erlang/tests/lists_ext.sx")
 (epoch 100)
 (eval "(list er-test-pass er-test-count)")
 (epoch 101)
@@ -87,6 +90,8 @@ cat > "$TMPFILE" << 'EPOCHS'
 (eval "(list er-vm-test-pass er-vm-test-count)")
 (epoch 111)
 (eval "(list er-sa-test-pass er-sa-test-count)")
+(epoch 112)
+(eval "(list er-lx-test-pass er-lx-test-count)")
 EPOCHS
 
 timeout 600 "$SX_SERVER" < "$TMPFILE" > "$OUTFILE" 2>&1