Harden safecsv against whitespace formula tricks

Ignore leading Unicode space when detecting formula cells,
extend starter runes, and document export usage in package
doc.

Signed-off-by: Cursor Agent <cursoragent@cursor.com>

Co-authored-by: Bryan FRIMIN <bryan@frimin.fr>

pkg/safecsv/cell.go

@@ -20,6 +20,12 @@
 
 package safecsv
 
+import (
+	"strings"
+	"unicode"
+	"unicode/utf8"
+)
+
 // SanitizeRecord returns a copy of record with spreadsheet-safe cell values.
 func SanitizeRecord(record []string) []string {
 	if len(record) == 0 {
@@ -35,15 +41,31 @@ func SanitizeRecord(record []string) []string {
 }
 
 // SanitizeCell prefixes values that spreadsheet tools may interpret as formulas.
+// Leading Unicode whitespace (including newlines) is ignored for detection only;
+// the written cell keeps the original text with a leading single-quote escape.
 func SanitizeCell(value string) string {
 	if value == "" {
 		return value
 	}
 
-	switch value[0] {
-	case '=', '+', '-', '@', '\t', '\r':
-		return "'" + value
-	default:
+	trimmed := strings.TrimLeftFunc(value, unicode.IsSpace)
+	if trimmed == "" {
 		return value
 	}
+
+	r, _ := utf8.DecodeRuneInString(trimmed)
+	if formulaLeadingRune(r) {
+		return "'" + value
+	}
+
+	return value
+}
+
+func formulaLeadingRune(r rune) bool {
+	switch r {
+	case '=', '+', '-', '@', '\\', '|', '%':
+		return true
+	default:
+		return false
+	}
 }