add ReferenceImage to the IR and use it for ![alt][ref]

The reference form of the image syntax had nowhere to go: Image only has a
url, so the parser was putting the label there and `![a][b]` came out
identical to `![a](b)`, with nothing downstream aware the url still needed
resolving against a RefDef.

Adds a ReferenceImage constructor mirroring ReferenceLink and has the djot
parser emit it. As with reference links, an empty label falls back to the
alt text.

Note HTML.hs still renders neither reference form.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/Djot.hs

@@ -893,9 +893,8 @@ image = do
         url <- inlineDestination
         pure $ Image {altText, url, title = Nothing, misc_attrs = mempty},
       do
-        -- the IR has no reference image, so the label stands in for the url
         label <- referenceLabel
-        pure $ Image {altText, url = if T.null label then altText else label, title = Nothing, misc_attrs = mempty}
+        pure $ ReferenceImage {altText, label = if T.null label then altText else label, attrs = mempty}
     ]
 
 -- | @(url)@, with line endings inside the url removed as djot specifies.
@@ -1000,6 +999,7 @@ plainOf = T.concat . map go
     go (Link {linkText}) = plainOf linkText
     go (ReferenceLink {linkText}) = plainOf linkText
     go (Image {altText}) = altText
+    go (ReferenceImage {altText}) = altText
     go (HTMLInline {inline_html_content}) = inline_html_content
     go (Superscript content _) = plainOf content
     go (Subscript content _) = plainOf content
@@ -1034,6 +1034,7 @@ applyAttrs element extra = case element of
   Link {linkText, url, title, misc_attrs} -> Link {linkText, url, title, misc_attrs = misc_attrs <> extra}
   ReferenceLink {linkText, label, attrs} -> ReferenceLink {linkText, label, attrs = attrs <> extra}
   Image {altText, url, title, misc_attrs} -> Image {altText, url, title, misc_attrs = misc_attrs <> extra}
+  ReferenceImage {altText, label, attrs} -> ReferenceImage {altText, label, attrs = attrs <> extra}
   FootnoteReference {label, attrs} -> FootnoteReference {label, attrs = attrs <> extra}
   -- anything else (plain text in particular) becomes a span carrying the attrs
   other -> Span [other] extra

src/IR.hs

@@ -90,6 +90,13 @@ data InlineText
         title :: Maybe Text,
         misc_attrs :: Attrs
       }
+  | -- the image equivalent of ReferenceLink, url comes from a RefDef with a
+    -- matching label
+    ReferenceImage
+      { altText :: Text,
+        label :: Text,
+        attrs :: Attrs
+      }
   | -- Markdown only DJOT uses RawInline
     HTMLInline {inline_html_content :: Text}
   | Superscript [InlineText] Attrs