Document Pilot Custom 823 nib variants

Documents the creator-used 14-karat gold Fine, Medium, and Broad nibs at the collection level. Explicitly states that individual recordings are not consistently attributable to a nib size and that line appearance alone must not be used to infer one. No schema or record metadata change.

Archive_Specifications.md

@@ -111,8 +111,12 @@ support or corrects this specification.
 ### Fountain pen
 
 - Pilot Custom 823
+- 14-karat gold nibs used in Fine, Medium, and Broad sizes
 - Aurora Black fountain pen ink
 
+The nib size used in an individual recording is not consistently documented and
+must not be inferred from line appearance alone.
+
 Additional tools and media are recorded at the individual-record level.
 
 ## Audio