Spell the alt rule in the flavour both directions read

spec/flavour.md

@@ -245,7 +245,8 @@ The moon, at night.
 
 **The CommonMark image.** A paragraph whose entire inline content is one image `![alt](url)` is
 a `mediaSingle` with attrs exactly `{"layout":"center"}` holding an `external` `media` — `url`
-from the destination, `alt` the description's plain-text content when non-empty. `adfToMarkdown`
+from the destination, `alt` the description's plain-text content when non-empty — a link or image
+inside it contributing its own text, and a break of either kind a space. `adfToMarkdown`
 emits the image form for exactly that shape — those attrs and no others, no marks on either
 node, no caption, and a `media` carrying nothing beyond `alt`, `type` and `url` — and only where
 CommonMark spells the pair: a destination or a description the image form cannot hold, an empty

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 ?? '')))
@@ -268,11 +267,17 @@ 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' }]
+  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[] {
   const runs: Run[] = []