documents: a right press in an open block targets the pressed byte

The open block's target was read at the selection's start and from
the syntax tree only, so a press on the second of two selected links
named the first, and an inline anchor read as its outer link.
point_open now reads the byte under the press and resolves links as
drawing does; delete_image takes that byte.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/documents/document/apart.rs

@@ -29,9 +29,10 @@ impl MarkdownDocument {
         )
     }
 
-    /// Removes the picture at the caret, read apart.
-    pub fn delete_image(&mut self, cx: &mut Context<Self>) -> Result<(), EditRefusal> {
-        self.apart("delete image".to_string(), drop_image, cx)
+    /// Removes the picture at source byte `at`, read apart.
+    pub fn delete_image(&mut self, at: usize, cx: &mut Context<Self>) -> Result<(), EditRefusal> {
+        let drop = move |full: &str, _| drop_image(full, at);
+        self.apart("delete image".to_string(), drop, cx)
     }
 
     /// `command` at the caret, read apart; refusals by event.