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.

1 files changed, 19 insertions, 3 deletions

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