fix(strutil): clamp Substr stop index to avoid out-of-range panic

Substr panicked with a slice-bounds error when a negative length trimmed
more runes than remained after pos, because stopIdx (strLn + length) could
fall before pos. Clamp stopIdx to pos so an over-long negative trim returns
an empty string, matching the existing pos >= strLn guard.

strutil/split.go

@@ -166,6 +166,12 @@ func Substr(s string, pos, length int) string {
 		stopIdx = strLn + length
 	}
 
+	// A negative length can move stopIdx before pos when it trims more than
+	// the remaining substring; clamp to pos to avoid an out-of-range slice.
+	if stopIdx < pos {
+		stopIdx = pos
+	}
+
 	return string(runes[pos:stopIdx])
 }