IxVM kernel: route list_lookup through list_drop

`list_lookup(list, idx)` previously walked the list inline (Cons + match
+ recurse). Replacing the body with `head(load(list_drop(list, idx)))`
lets `list_drop`'s memo dedup sublist pointers — content-addressed
intermediates collapse across all lookups that pass through them.

`lake exe check`:
* Nat.add_comm:                                  54_670_728 → 54_350_107 FFT (-0.6%)
* `_private....utf8DecodeChar?.helper₃`:    38_696_601_688 → 37_793_682_359 FFT (-2.3%)

Author: arthurpaulino

Ix/IxVM/Core.lean

@@ -52,11 +52,8 @@ def core := ⟦
   }
 
   fn list_lookup‹T›(list: List‹T›, idx: G) -> T {
-    let ListNode.Cons(v, rest) = load(list);
-    match idx {
-      0 => v,
-      _ => list_lookup(rest, idx - 1),
-    }
+    let ListNode.Cons(v, _) = load(list_drop(list, idx));
+    v
   }
 
   fn list_lookup_u64‹T›(list: List‹T›, idx: U64) -> T {