Fix UUID heuristic to correctly identify pure-hex strings

The previous check `not t.isalpha()` incorrectly rejects valid 8-char
UUIDs composed entirely of A-F characters (e.g. DEADBEEF). MFC UUIDs are
CRC32-based 8-char uppercase hex strings, so checking isalpha() introduces
false negatives for any UUID whose digits happen to all be A-F.

Fix by checking that every character is a valid hex digit using:
  all(c in '0123456789abcdef' for c in t.lower())

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: sbryngelson

toolchain/mfc/test/test.py

@@ -68,7 +68,7 @@ def __filter(cases_) -> typing.List[TestCase]:
     if len(ARG("only")) > 0:
         # UUIDs are 8-char hex (CRC32): use OR so --only UUID1 UUID2 selects
         # any matching test. Labels use AND so --only 2D Bubbles selects both.
-        _uuid_mode = all(len(t) == 8 and t.isalnum() and not t.isalpha() for t in ARG("only"))
+        _uuid_mode = all(len(t) == 8 and all(c in '0123456789abcdef' for c in t.lower()) for t in ARG("only"))
         for case in cases[:]:
             case: TestCase