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/lists-ext.sx

@@ -319,6 +319,53 @@
       (er-mk-tuple
         (list (er-ext-takewhile pred lst) (er-ext-dropwhile pred lst))))))
 
+;; ── structural / aggregate (flatten / max / min) ──────────────────
+(define
+  er-ext-flatten
+  (fn (lst)
+    (cond
+      (er-nil? lst) (er-mk-nil)
+      (er-cons? lst)
+        (let ((h (get lst :head)))
+          (if (or (er-nil? h) (er-cons? h))
+            (er-list-append (er-ext-flatten h) (er-ext-flatten (get lst :tail)))
+            (er-mk-cons h (er-ext-flatten (get lst :tail)))))
+      :else (raise (er-mk-error-marker (er-mk-atom "badarg"))))))
+
+(define
+  er-bif-lists-flatten
+  (fn (vs) (er-ext-flatten (er-bif-arg1 vs "lists:flatten"))))
+
+(define
+  er-ext-extreme
+  (fn (lst best lt?)
+    (cond
+      (er-nil? lst) best
+      (er-cons? lst)
+        (er-ext-extreme
+          (get lst :tail)
+          (if (lt? best (get lst :head)) (get lst :head) best)
+          lt?)
+      :else best)))
+
+(define
+  er-bif-lists-max
+  (fn (vs)
+    (let ((lst (er-bif-arg1 vs "lists:max")))
+      (if (er-cons? lst)
+        (er-ext-extreme (get lst :tail) (get lst :head)
+          (fn (a b) (er-ext-lt? a b)))
+        (raise (er-mk-error-marker (er-mk-atom "badarg")))))))
+
+(define
+  er-bif-lists-min
+  (fn (vs)
+    (let ((lst (er-bif-arg1 vs "lists:min")))
+      (if (er-cons? lst)
+        (er-ext-extreme (get lst :tail) (get lst :head)
+          (fn (a b) (er-ext-lt? b a)))
+        (raise (er-mk-error-marker (er-mk-atom "badarg")))))))
+
 ;; ── register ──────────────────────────────────────────────────────
 ;; Hook into er-register-builtin-bifs! rather than registering once:
 ;; the registry can be reset + rebuilt mid-run (tests/runtime.sx does
@@ -340,7 +387,10 @@
     (er-register-pure-bif! "lists" "partition" 2 er-bif-lists-partition)
     (er-register-pure-bif! "lists" "takewhile" 2 er-bif-lists-takewhile)
     (er-register-pure-bif! "lists" "dropwhile" 2 er-bif-lists-dropwhile)
-    (er-register-pure-bif! "lists" "splitwith" 2 er-bif-lists-splitwith)))
+    (er-register-pure-bif! "lists" "splitwith" 2 er-bif-lists-splitwith)
+    (er-register-pure-bif! "lists" "flatten" 1 er-bif-lists-flatten)
+    (er-register-pure-bif! "lists" "max" 1 er-bif-lists-max)
+    (er-register-pure-bif! "lists" "min" 1 er-bif-lists-min)))
 
 (define er-ext-prev-register-builtins er-register-builtin-bifs!)
 (define er-register-builtin-bifs!