Implements option to generate prefix-free hints

- Declares --prefix-free CLI flag in main.py.
- Implements dynamic index offset shifting in marks.go to prevent
  generating hints that prefix other hints.
- Integrates prefix-free encoding in main.go.
- Includes tests.

Implements #10195

kittens/hints/marks.go

@@ -706,13 +706,28 @@ process_answer:
 	}
 	largest_index := ans[len(ans)-1].Index
 	offset := max(0, opts.HintsOffset)
+	if opts.PrefixFree {
+		alphabetLength := len(opts.Alphabet)
+		if alphabetLength == 0 {
+			alphabetLength = len(DEFAULT_HINT_ALPHABET)
+		}
+		offset = max(offset, hints_to_skip(len(ans), alphabetLength))
+	}
 	index_map = make(map[int]*Mark, len(ans))
 	for i := range ans {
 		m := &ans[i]
-		if opts.Ascending {
-			m.Index += offset
+		if opts.PrefixFree {
+			if opts.Ascending {
+				m.Index = i + offset + 1
+			} else {
+				m.Index = (largest_index - m.Index) + offset + 1
+			}
 		} else {
-			m.Index = largest_index - m.Index + offset
+			if opts.Ascending {
+				m.Index += offset
+			} else {
+				m.Index = largest_index - m.Index + offset
+			}
 		}
 		index_map[m.Index] = m
 	}