Rewrite build_lfn_short_entry_name() and add _gen_numname_suffix() helper
to match the gen_numname() algorithm in ff.c. This fixes:
- chr(order) producing raw binary instead of ASCII digits
- Collision for order >= 10 when str(order) makes the name exceed 8 chars
- Hex suffix with dynamic stem shortening (matching C implementation)
- CRC16-CCITT hash for seq > 5 to reduce collision probability
Also fix LDIR_Name2_SIZE typo in long_filename_utils.py (should be
LDIR_Name3_SIZE), which made the assertion guard too permissive.
Add ShortFilenameGenerationTestCase with 9 unit tests covering single-digit,
multi-digit, hash-based, and collision-free generation scenarios.
Adam Múdry
Martin Gaňo