Spell the alt rule in the flavour both directions read

src/markdown/parse/inline-content.ts

@@ -250,7 +250,6 @@ function closeImage(scan: Scan, at: number, inner: readonly Piece[], definition:
   return success(null)
 }
 
-// spec/flavour.md: the description's plain-text content, where a break of either kind reads as a space.
 function imageAlt(inner: readonly Piece[]): string {
   return resolveNodes(inner)
     .map((node) => (node.type === 'hardBreak' ? ' ' : (node.text ?? '')))
@@ -268,10 +267,16 @@ function resolveNodes(pieces: readonly Piece[]): AdfNode[] {
 
 // Only `imageAlt` reaches the image arm: everywhere else an image amid other content is refused first.
 function pieceNodes(piece: Piece): AdfNode[] {
-  if (piece.kind === 'nodes') return piece.nodes
-  if (piece.kind === 'open') return bracketNodes(piece)
-  if (piece.kind === 'image') return piece.alt === '' ? [] : [{ text: piece.alt, type: 'text' }]
-  return []
+  switch (piece.kind) {
+    case 'image':
+      return piece.alt === '' ? [] : [{ text: piece.alt, type: 'text' }]
+    case 'nodes':
+      return piece.nodes
+    case 'open':
+      return bracketNodes(piece)
+    case 'run':
+      return []
+  }
 }
 
 function delimiterRuns(pieces: readonly Piece[]): Run[] {