Fixed KeyError on bare crc16 and crc codecs

[gaoflow]

codext.encode(data, "crc16") — the most common CRC width — raises KeyError: '', and so does a bare crc, while every other width resolves fine with a bare name:

>>> codext.encode(b"123456789", "crc16")
KeyError: ''
>>> codext.encode(b"123456789", "crc8")
b'f4'

Root cause: in checksums/crc.py a bare crcN resolves to the '' key of that width's variant table. Every width defines that '' default except width 16, and the top-level crc-a table only has 'a' — so those two are the only bare names that crash.

I checked the rest of the table is sound: crc("123456789", …) matches the stored check value for all 162 registered variants (0 mismatches), so the only defect is the two missing defaults. This adds them:

• crc16 → CRC-16/ARC (poly 0x8005) = reveng's canonical "CRC-16", identical to the existing arc/ibm/lha entries → bb3d
• crc → CRC-A, identical to crca → bf05

The test extends the existing checksum test with an invariant that every registered crc<width> (and bare crc) resolves without a variant suffix and equals its default check, so a future width added without a '' default would be caught.

If you'd rather keep bare crc16 explicit, the alternative is to raise a clear ValueError ("ambiguous, specify a variant") instead of KeyError — I defaulted it to stay consistent with the other 16 widths.

[mergify]

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