docs: consolidate design change logs and refresh stale docs

docs/design/states.md

@@ -108,7 +108,7 @@ Schema changes with entries: Save opens the review ("Add 2 fields, change 1, rem
 Entry rules, 422 except unique (409): required, type, min, max, options, unique, unknown_field. Messages are per type and drawn in the field-type gallery (group "Field types").
 
 ## Media field (pass 5)
-Copy and states are in `media-field-changes.md` (sections 2 to 6). Entry rules for a media field: required 422 "Pick an image for Thumbnail."; type 422 "Thumbnail must be an image from the library."; min, max, options and unique do not apply. A required media field added to a type with entries is refused client side in the review ("Can't be applied") and by the server as 409 schema_incompatible. Image deleted while entries use it: the control shows "This image is no longer in the library"; saving is refused only when the field is required. HTTP statuses for the picker and the field use the table above (400, 401, 403, 404, 409, 413, 429, 500, network).
+Copy and states are in `media-field.md` (sections 2 to 6). Entry rules for a media field: required 422 "Pick an image for Thumbnail."; type 422 "Thumbnail must be an image from the library."; min, max, options and unique do not apply. A required media field added to a type with entries is refused client side in the review ("Can't be applied") and by the server as 409 schema_incompatible. Image deleted while entries use it: the control shows "This image is no longer in the library"; saving is refused only when the field is required. HTTP statuses for the picker and the field use the table above (400, 401, 403, 404, 409, 413, 429, 500, network).
 
 ## Roles and permissions