irmin-lwt: expose Tree.inspect, counters, dump_counters, reset_counters, merge

These were missing from the [Tree] submodule of [module type S]:

- [val merge : t Irmin.Merge.t]: the merge value used by Irmin tree
  combinators.
- [type counters], [val counters], [val dump_counters],
  [val reset_counters]: the performance counters shared between all
  trees backed by the same set of internal caches. [counters] is kept
  abstract since it is a record (nominal typing) and the underlying
  [S.Tree.counters] is abstract from inside the functor body.
- [val inspect]: a synchronous inspector returning a transparent variant
  describing the kind and internal state of a tree node ([`Map],
  [`Key], [`Value], [`Portable_dirty], [`Pruned]).

[helpers/context.ml] in Octez calls [inspect] for debugging /
introspection paths.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: balat

src/irmin-lwt/irmin_lwt.ml

@@ -597,6 +597,19 @@ module type S = sig
       t ->
       'a ->
       'a Lwt.t
+
+    val merge : t Irmin.Merge.t
+
+    type counters
+
+    val counters : unit -> counters
+    val dump_counters : unit Fmt.t
+    val reset_counters : unit -> unit
+
+    val inspect :
+      t ->
+      [ `Contents
+      | `Node of [ `Map | `Key | `Value | `Portable_dirty | `Pruned ] ]
   end
 
   (** {1 Commits} *)
@@ -1134,6 +1147,15 @@ module Make (S : Irmin.Generic_key.S) = struct
       run_eio (fun () ->
           S.Tree.fold ?order ?force ?cache ?uniq ?pre ?post ?depth ?contents
             ?node ?tree t acc)
+
+    let merge = S.Tree.merge
+
+    type counters = S.Tree.counters
+
+    let counters = S.Tree.counters
+    let dump_counters = S.Tree.dump_counters
+    let reset_counters = S.Tree.reset_counters
+    let inspect = S.Tree.inspect
   end
 
   module Commit = struct

src/irmin-lwt/irmin_lwt.mli

@@ -616,6 +616,25 @@ module type S = sig
       t ->
       'a ->
       'a Lwt.t
+
+    (** {2 Merge} *)
+
+    val merge : t Irmin.Merge.t
+
+    (** {2 Performance counters and inspection} *)
+
+    type counters
+
+    val counters : unit -> counters
+    val dump_counters : unit Fmt.t
+    val reset_counters : unit -> unit
+
+    val inspect :
+      t ->
+      [ `Contents
+      | `Node of [ `Map | `Key | `Value | `Portable_dirty | `Pruned ] ]
+    (** [inspect t] is similar to {!val-kind}, with extra state information
+        returned for nodes. Pure: no I/O. *)
   end
 
   (** {1 Commits} *)