fix(naming): config fix round 1 -- unknown sections, O(n) accumulate

F1: test_unknown_key_is_reported_not_fatal never asserted unknown_keys
itself, only that parsing survives -- a no-op accumulator passed it.
Now asserts the key is actually collected.

F2: a misspelled section name, e.g. [deafults], was silently discarded --
Ok empty, lang and everything else gone, nothing reported. That is the
highest-value typo this feature exists to catch. Any section other than
[defaults] is now collected into a new Config.unknown_sections, kept
separate from unknown_keys so the CLI can word the two warnings
differently. Still non-fatal: a newer colitur's added section must not
break an older binary.

F3: overlays and unknown_keys accumulated with '@ [v]' per line, O(n^2)
over the field count. Cons during the fold, List.rev once at the end.

F4: documented that lang/template/format are last-wins on a repeated
key, the opposite direction from Overlay_ini.get's first-wins over the
same section type.

2 files changed, 56 insertions, 21 deletions

diff --git a/lib/naming/config.ml b/lib/naming/config.ml
index ce1b433..0fafd1f 100644
--- a/lib/naming/config.ml
+++ b/lib/naming/config.ml
@@ -6,36 +6,55 @@ type t = {
   template : string option;
   format : string option;
   unknown_keys : string list;
+  unknown_sections : string list;
 }
 
-let empty = { lang = None; overlays = []; template = None; format = None; unknown_keys = [] }
+let empty =
+  { lang = None; overlays = []; template = None; format = None; unknown_keys = [];
+    unknown_sections = [] }
 
 let lang t = t.lang
 let overlays t = t.overlays
 let template t = t.template
 let format t = t.format
 let unknown_keys t = t.unknown_keys
+let unknown_sections t = t.unknown_sections
 
 let of_string text =
   match OI.parse_sections text with
   | Error e -> Error e
-  | Ok sections -> (
-      match List.find_opt (fun (s : OI.section) -> s.OI.name = "defaults") sections with
-      | None -> Ok empty
-      | Some s ->
-          let acc =
-            List.fold_left
-              (fun acc (k, v) ->
-                match k with
-                | "lang" -> { acc with lang = Some v }
-                | "template" -> { acc with template = Some v }
-                | "format" -> { acc with format = Some v }
-                (* accumulates: a user has more than one overlay *)
-                | "overlay" -> { acc with overlays = acc.overlays @ [ v ] }
-                | other -> { acc with unknown_keys = acc.unknown_keys @ [ other ] })
-              empty s.OI.fields
-          in
-          Ok acc)
+  | Ok sections ->
+      (* Any section that is not [defaults] is unrecognised -- including a
+         plain typo such as [deafults] -- and must be REPORTED, never
+         silently dropped: that is precisely the failure this feature exists
+         to surface. *)
+      let unknown_sections =
+        List.filter_map
+          (fun (s : OI.section) -> if s.OI.name = "defaults" then None else Some s.OI.name)
+          sections
+      in
+      let acc =
+        match List.find_opt (fun (s : OI.section) -> s.OI.name = "defaults") sections with
+        | None -> empty
+        | Some s ->
+            (* Cons then reverse once at the end, not `@ [v]` per line: the
+               latter is O(n^2) over the field count, a real hang on a
+               machine-generated file with many overlay lines. *)
+            let acc =
+              List.fold_left
+                (fun acc (k, v) ->
+                  match k with
+                  | "lang" -> { acc with lang = Some v }
+                  | "template" -> { acc with template = Some v }
+                  | "format" -> { acc with format = Some v }
+                  (* accumulates: a user has more than one overlay *)
+                  | "overlay" -> { acc with overlays = v :: acc.overlays }
+                  | other -> { acc with unknown_keys = other :: acc.unknown_keys })
+                empty s.OI.fields
+            in
+            { acc with overlays = List.rev acc.overlays; unknown_keys = List.rev acc.unknown_keys }
+      in
+      Ok { acc with unknown_sections }
 
 let resolve ~flag ~config ~default =
   match flag with
diff --git a/lib/naming/config.mli b/lib/naming/config.mli
index 41ff429..27f0c44 100644
--- a/lib/naming/config.mli
+++ b/lib/naming/config.mli
@@ -13,16 +13,32 @@ type t
 val empty : t
 val of_string : string -> (t, string) result
 
+(** [lang], [template] and [format] are each set from a single field. A
+    repeated key is LAST-WINS -- the opposite direction from
+    {!Colitur_kernel.Overlay_ini.get}'s first-wins over the same [section]
+    type -- because the natural reading of a config file a user edited by
+    hand and appended to is "the bottom line is the one that took effect". *)
 val lang : t -> string option
+
 val overlays : t -> string list
 val template : t -> string option
 val format : t -> string option
 
-(** Keys present in the file that this build does not understand. Reported, never
-    fatal: a config written for a newer colitur must still work on an older one,
-    but silently ignoring a line the user wrote is how a typo becomes invisible. *)
+(** Keys present in the [\[defaults\]] section that this build does not
+    understand. Reported, never fatal: a config written for a newer colitur
+    must still work on an older one, but silently ignoring a line the user
+    wrote is how a typo becomes invisible. *)
 val unknown_keys : t -> string list
 
+(** Section names other than [\[defaults\]], reported separately from
+    {!unknown_keys} so the CLI can word the two warnings differently (a
+    misspelled section, e.g. [\[deafults\]], versus a misspelled key inside a
+    recognised one). Also never fatal, and never silent: a section this build
+    does not recognise is exactly the highest-value typo this feature exists
+    to catch, because it silently discards the whole section -- [lang] and
+    everything else in it -- with no other way for the user to notice. *)
+val unknown_sections : t -> string list
+
 (** [resolve ~flag ~config ~default] returns [(value, source)] with source one of
     ["flag"], ["config"], ["default"]. Precedence is flag > config > default. *)
 val resolve : flag:string option -> config:string option -> default:string -> string * string