exp/textinput: add Field.HasText

Closes #3419

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

exp/textinput/field.go

@@ -281,6 +281,11 @@ func (f *Field) TextForRendering() string {
 	return b.String()
 }
 
+// HasText reports whether the field has any text.
+func (f *Field) HasText() bool {
+	return f.pieceTable.hasText()
+}
+
 // TextLengthInBytes returns the length of the current text in bytes.
 func (f *Field) TextLengthInBytes() int {
 	return f.pieceTable.Len()

exp/textinput/piecetable.go

@@ -139,6 +139,15 @@ func (p *pieceTable) writeToWithInsertion(w io.Writer, text string, start, end i
 	return n, nil
 }
 
+func (p *pieceTable) hasText() bool {
+	for _, item := range p.items() {
+		if item.start < item.end {
+			return true
+		}
+	}
+	return false
+}
+
 func (p *pieceTable) Len() int {
 	var n int
 	items := p.items()