Expand embedded objects in braille document lines

Structural navigation to a heading whose native text is a single embedded-object placeholder for its child link showed only that placeholder in braille while speech used the expanded title. Braille line reads now expand embedded objects like speech capture, preserving RawText and native offsets; expanded lines carry no cell routes or native selection/attribute dots and only a line-start cursor. Frame assembly strips any remaining placeholders and falls back to displayed text so a placeholder never reaches the display. Regression tests cover placeholder-only, mixed, expanded, assembled, and empty headings plus native linked-heading and Firefox mixed-line braille. Maintainer confirms physical braille and speech agree on YouTube headings.

internal/atspi/document_caret.go

@@ -22,12 +22,17 @@ func (unit DocumentCaretUnit) Valid() bool {
 }
 
 type DocumentCaretText struct {
-	TextAttributeRuns            []TextAttributeRun
-	Start                        int
-	End                          int
-	Text                         string
-	RawText                      string
-	Misspelled                   bool
+	TextAttributeRuns []TextAttributeRun
+	Start             int
+	End               int
+	Text              string
+	RawText           string
+	Misspelled        bool
+	// Expanded reports that embedded-object placeholders in Text were
+	// replaced by their descendant text. Start, End, selections, attribute
+	// runs, and RawText still describe the unexpanded native line, so
+	// native offsets no longer map one-to-one into Text.
+	Expanded                     bool
 	CharacterCount               int
 	SelectionStart, SelectionEnd int
 	// Bounds describe this native line, including offscreen document text.