erlang: lists flatten/1 + max/1 + min/1 (833/833)

Structural/aggregate ops in lib/erlang/lists-ext.sx: flatten/1 deep
flatten, max/1 and min/1 by full Erlang term order (badarg on empty).
Extreme-finder uses er-ext-lt?'s SX boolean directly in if (er-truthy?
only recognises Erlang bool atoms). lists_ext suite 52 -> 62.

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

lib/erlang/tests/lists_ext.sx

@@ -205,3 +205,36 @@
 
 (er-lx-test "splitwith empty"
   (er-lx-nm "lists:splitwith(fun(_) -> true end, []) =:= {[],[]}") "true")
+
+;; ── lists:flatten/1 ───────────────────────────────────────────────
+(er-lx-test "flatten nested"
+  (er-lx-nm "lists:flatten([1,[2,[3,4]],5]) =:= [1,2,3,4,5]") "true")
+
+(er-lx-test "flatten already flat"
+  (er-lx-nm "lists:flatten([1,2,3]) =:= [1,2,3]") "true")
+
+(er-lx-test "flatten empty"
+  (er-lx-nm "lists:flatten([]) =:= []") "true")
+
+(er-lx-test "flatten deep empties"
+  (er-lx-nm "lists:flatten([[],[1],[[]]]) =:= [1]") "true")
+
+(er-lx-test "flatten length"
+  (erlang-eval-ast "length(lists:flatten([[1,2],[3],[4,5,6]]))") 6)
+
+;; ── lists:max/1 ───────────────────────────────────────────────────
+(er-lx-test "max ints"
+  (erlang-eval-ast "lists:max([3,1,4,1,5,9,2,6])") 9)
+
+(er-lx-test "max single"
+  (erlang-eval-ast "lists:max([7])") 7)
+
+(er-lx-test "max atoms term order"
+  (er-lx-nm "lists:max([a,c,b]) =:= c") "true")
+
+;; ── lists:min/1 ───────────────────────────────────────────────────
+(er-lx-test "min ints"
+  (erlang-eval-ast "lists:min([3,1,4,1,5])") 1)
+
+(er-lx-test "min mixed term order"
+  (er-lx-nm "lists:min([a,1,b]) =:= 1") "true")