Route Lambda.apply's eta reduction through the folding constructors

lambda.mli says a term is built through the constructors, seven of which
normalize as they build. apply did not: its eta reduction substituted the
call's arguments into the inner primitive and then rebuilt the result with a
raw Lprim, so a primitive applied to constants was left unfolded.

    ((a, b) => a + b)(1, 2)

left translation as (+ 1 2) rather than 3. It could only do this by owning
the type; the reason it did was position, which the preceding commit fixed.

Generated JavaScript is unchanged: the optimizer passes were folding this on
their way past, so the same code comes out. It now happens once, at
construction, instead of being rediscovered on every full-tree rebuild.

Two of the three raw constructions in lambda.ml remain - offset_ref and
mk_builtin. Neither is a tidy-up: mk_builtin makes constant guards fold at
production, which is only safe since guards became data, and offset_ref is
untested.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W8g8qwBARAcvW9MyuKQq8H

Author: cristianoc

compiler/ml/lambda.ml

@@ -1043,8 +1043,9 @@ let rec apply ?(ap_transformed_jsx = false) fn args (ap_info : ap_info) : t =
     match is_eta_conversion_exn params inner_args args with
     | args ->
       let loc = ap_info.ap_loc in
-      Lprim
-        {primitive = wrap; args = [Lprim {primitive_call with args; loc}]; loc}
+      prim ~primitive:wrap
+        ~args:[prim ~primitive:primitive_call.primitive ~args loc]
+        loc
     | exception Not_simple_form ->
       Lapply {ap_func = fn; ap_args = args; ap_info; ap_transformed_jsx})
   | Lfunction
@@ -1053,7 +1054,7 @@ let rec apply ?(ap_transformed_jsx = false) fn args (ap_info : ap_info) : t =
         body = Lprim ({primitive = _; args = inner_args} as primitive_call);
       } -> (
     match is_eta_conversion_exn params inner_args args with
-    | args -> Lprim {primitive_call with args; loc = ap_info.ap_loc}
+    | args -> prim ~primitive:primitive_call.primitive ~args ap_info.ap_loc
     | exception _ ->
       Lapply {ap_func = fn; ap_args = args; ap_info; ap_transformed_jsx})
   | Lfunction
@@ -1066,7 +1067,7 @@ let rec apply ?(ap_transformed_jsx = false) fn args (ap_info : ap_info) : t =
       } -> (
     match is_eta_conversion_exn params inner_args args with
     | args ->
-      Lsequence (Lprim {primitive_call with args; loc = ap_info.ap_loc}, const)
+      seq (prim ~primitive:primitive_call.primitive ~args ap_info.ap_loc) const
     | exception _ ->
       Lapply {ap_func = fn; ap_args = args; ap_info; ap_transformed_jsx}
       (* | Lfunction {params;body} when Ext_list.same_length params args ->
@@ -1075,7 +1076,7 @@ let rec apply ?(ap_transformed_jsx = false) fn args (ap_info : ap_info) : t =
           ) params args body *)
       (* TODO: more rigirous analysis on [let_kind] *))
   | Llet (kind, id, e, (Lfunction _ as fn)) ->
-    Llet (kind, id, e, apply fn args ap_info ~ap_transformed_jsx)
+    let_ kind id e (apply fn args ap_info ~ap_transformed_jsx)
   (* | Llet (kind0, id0, e0, Llet (kind,id, e, (Lfunction _ as fn))) ->
      Llet(kind0,id0,e0,Llet (kind, id, e, apply fn args loc status)) *)
   | _ -> Lapply {ap_func = fn; ap_args = args; ap_info; ap_transformed_jsx}