fix: the CSV round trip keeps values that start with an apostrophe before a formula character (WR-21)

escapeCell left '=x as it was and unescapeCell then dropped its
apostrophe on import. A value of apostrophes followed by a formula
character now gets one more apostrophe on export, and the import drops
exactly one, so unescapeCell is the inverse of escapeCell.

classes/csv.go

@@ -38,19 +38,27 @@ func formulaStart(s string) bool {
 	return false
 }
 
+// escapable reports whether s is a formula cell behind any number of
+// apostrophes: the cells escapeCell prefixes.
+func escapable(s string) bool {
+	return formulaStart(strings.TrimLeft(s, "'"))
+}
+
 // escapeCell prefixes a formula cell with an apostrophe (CSV formula
-// injection, T-32-24).
+// injection, T-32-24). A value that already starts with apostrophes before
+// a formula character gets one more, so unescapeCell is its exact inverse
+// and '=x survives a round trip (WR-21).
 func escapeCell(s string) string {
-	if formulaStart(s) {
+	if escapable(s) {
 		return "'" + s
 	}
 	return s
 }
 
-// unescapeCell drops the apostrophe escapeCell added, so an exported file
-// imports unchanged.
+// unescapeCell drops the one apostrophe escapeCell added, so an exported
+// file imports unchanged.
 func unescapeCell(s string) string {
-	if strings.HasPrefix(s, "'") && formulaStart(s[1:]) {
+	if strings.HasPrefix(s, "'") && escapable(s[1:]) {
 		return s[1:]
 	}
 	return s