docs: consolidate design change logs and refresh stale docs

features/media_field_spa.feature

@@ -7,7 +7,7 @@ Feature: Media field type in the SPA
     Given the SPA calls the API with relative URLs under /v1 and the "jwt" HttpOnly cookie
     And API errors are plain text
     And this slice changes only web/ (and web/visual); the backend of features/media_field_type.feature is done
-    And the design is docs/design/media-field-changes.md sections 1, 2, 3.2 to 3.7, 4, 6 and 7, with the board groups 2 Schema builder, 3 Review and delete, 4 Field types, 5 Content and entry list, 6 Entry editor and 10 Media picker for fields of "RenewCMS Screens Schema.dc.html"
+    And the design is docs/design/media-field.md sections 1, 2, 3.2 to 3.7, 4 and 6, with the board groups 2 Schema builder, 3 Review and delete, 4 Field types, 5 Content and entry list, 6 Entry editor and 10 Media picker for fields of "RenewCMS Screens Schema.dc.html"
     And the registry is lib/fieldTypes with lib/entryEditors, the single source for the builder, the list cells and the entry form; no screen branches on the type name "media" where a registry property can say it
     And a media value is the id of a library image (a uint32 of at least 1) or null
     And every entry answer (POST 201, PUT 200, GET one, each list item) carries a top-level "media" object keyed by media field key: {"id","filename","width","height","size","thumbnail_url"} for an existing image, or {"id","missing":true} when the image is gone; a null value has no key
@@ -476,7 +476,7 @@ Feature: Media field type in the SPA
     Given the card or a title needs the text of a media cell
     Then it reads "–" for none and never the numeric id
 
-  # ---------- Accessibility (media-field-changes.md section 6)
+  # ---------- Accessibility (media-field.md section 6)
 
   Scenario: Accessibility of the control
     Then the control is role="group" labelled by the field label