Suppress hypertext object markers in text-change speech

Thunderbird's message composer emits U+FFFC object replacement characters
when creating or removing paragraph objects. Treating these structural
updates as inserted or deleted text sent raw placeholders to speech,
including multi-character updates that bypass ordinary key-echo settings.

Remove object markers from insertion and deletion presentation while
preserving surrounding text. Normalize typing-echo payloads before deciding
the echo kind or applying character, word, and sentence echo policies.
Return no text or echo kind for updates containing only object markers.

Add regression cases for single and repeated markers, mixed marker/text
payloads, and ordinary text through both event presentation and typing echo.

Validation:
- go test ./... -count=1, with the local cache and dependency network disabled.
- Built the reader and the BrlAPI-tagged Braille helper.
- A disposable Thunderbird profile on a private X11 display and session bus
  produced one marker-containing presentation before the fix and none after.
- Private Thunderbird checks preserved character, word, and line navigation
  after typing several paragraphs.
- The user confirmed main-arrow reading worked with the restarted build.

The original silent-arrow failure was not reproduced in isolation. Restarting
restored reading in the user's session; this commit does not claim to fix
its underlying cause or change arrow-key handling. Physical Braille output
was not tested. Preserve the original and fresh diagnostic logs locally.

internal/presentation/presentation.go

@@ -256,6 +256,9 @@ func EventSpeechWithOptions(event atspi.Event, options Options) Speech {
 			return Speech{Text: event.Source.Text}
 		}
 	case atspi.TextInsertedEvent:
+		// Hypertext editors report paragraph creation as inserted object
+		// placeholders. These describe structure rather than typed text.
+		event.Payload = strings.ReplaceAll(event.Payload, "\ufffc", "")
 		if !shouldEchoInsertion(event.Payload, insertionEchoOptions(event, options)) {
 			return Speech{}
 		}
@@ -264,6 +267,7 @@ func EventSpeechWithOptions(event atspi.Event, options Options) Speech {
 		}
 		return Speech{Text: event.Payload}
 	case atspi.TextDeletedEvent:
+		event.Payload = strings.ReplaceAll(event.Payload, "\ufffc", "")
 		if utf8.RuneCountInString(event.Payload) == 1 && unicode.IsSpace(firstRune(event.Payload)) {
 			return Speech{Text: caretCharacterText(event.Payload)}
 		}

internal/presentation/typing_echo.go

@@ -31,6 +31,12 @@ func TypingEchoText(event atspi.Event, previousText string, options Options) str
 }
 
 func TypingEcho(event atspi.Event, previousText string, options Options) (string, string) {
+	if event.Type == atspi.TextInsertedEvent || event.Type == atspi.TextDeletedEvent {
+		event.Payload = strings.ReplaceAll(event.Payload, "\ufffc", "")
+		if event.Payload == "" {
+			return "", ""
+		}
+	}
 	options = insertionEchoOptions(event, options)
 	kind := ""
 	if event.Type == atspi.TextInsertedEvent && utf8.RuneCountInString(event.Payload) == 1 {

internal/presentation/typing_echo_test.go

@@ -164,3 +164,26 @@ func TestTypingEchoInEditFieldsOnly(t *testing.T) {
 		})
 	}
 }
+
+func TestEditorStructuralChangesDoNotSpeakObjectMarkers(t *testing.T) {
+	for _, eventType := range []string{atspi.TextInsertedEvent, atspi.TextDeletedEvent} {
+		for _, tc := range []struct{ payload, want string }{
+			{"\ufffc", ""},
+			{"\ufffc\ufffc", ""},
+			{"\ufffcDraft\ufffc", "Draft"},
+			{"ordinary text", "ordinary text"},
+		} {
+			t.Run(eventType+tc.payload, func(t *testing.T) {
+				event := atspi.NewEventWithPayload(eventType, atspi.Snapshot{
+					Role: "document web", Editable: true,
+				}, tc.payload)
+				if got := EventTextWithOptions(event, DefaultOptions()); got != tc.want {
+					t.Fatalf("structural change speech = %q, want %q", got, tc.want)
+				}
+				if got := TypingEchoText(event, "", DefaultOptions()); got != tc.want {
+					t.Fatalf("structural change echo = %q, want %q", got, tc.want)
+				}
+			})
+		}
+	}
+}