NaturalStringComparer is intransitive for non-ASCII digits (٥ < 10 < z < ٥), so sort output depends on input order #55

Follow-up to #50 / #51. That fix made two numeric chunks compare by value, whatever script their digits are in. It left two gaps.

1. A digit chunk compared with a text chunk still uses raw code points, which creates a cycle

When exactly one chunk is numeric (one string starts with a digit, the other with a non-digit), Compare falls into the else branch (Sorting/NaturalStringComparer.cs:~76) and uses string.Compare(..., Ordinal).

• ASCII digits (U+0030–0039) sort before letters ordinally.
• Non-ASCII digits such as Arabic-Indic ٥ (U+0665) sort after them.

Combined with the numeric path, which says ٥ == 5 and ٥ < 10, this breaks transitivity.

Reproduction (ran):

• Compare("٥", "10") < 0, Compare("10", "z") < 0, and Compare("z", "٥") < 0. That is a cycle.
• Compare("٥", "5") == 0, yet "5" sorts before "z" while "٥" sorts after it.
• List.Sort of {"٥","10","z"} gives z, ٥, 10, but {"z","10","٥"} gives ٥, 10, z.

Why it matters: An intransitive IComparer breaks the contract List.Sort, OrderBy and SortedSet rely on. Results become order-dependent, and SortedSet/SortedDictionary can fail lookups.

2. Digits outside the BMP are still compared as text

The class remarks say any run of category Nd digits is a number "whatever script it is written in". However, the \d+ regex and char.IsDigit(Value[0]) work on UTF-16 code units, and surrogate halves are category Cs. So these digits never form a numeric chunk:

• Mathematical digits U+1D7CE–1D7FF
• Adlam
• Osmanya

Reproduction (ran): Compare("file𝟗", "file𝟏𝟎") returns positive (9 > 10), while Compare("file9", "file10") returns negative.

Suggested fix / acceptance criteria

• Mixed numeric/text chunks: return a fixed order (for example, numeric before text, which matches the ASCII behaviour) instead of an ordinal comparison.
• Chunking: chunk by code point (Rune enumeration, Rune.IsDigit, Rune.GetNumericValue) rather than by char. Alternatively, narrow the doc comment to BMP digits.
• Tests:
  • A property test that Compare is transitive over a mixed set of ASCII/non-ASCII digit strings and letters.
  • "file𝟗" < "file𝟏𝟎".