Add BLAKE2 b2sum support

[sarsolot]

Closes #9

Summary

Add BLAKE2 support as a single b2 type, following the same shape as b3 (#88).

• Single b2 type with --length for variable digest size in bits (default 512, max 512)
• Uses Python's built-in hashlib.blake2b - no external dependency
• Recognized extensions: .b2 (default), .blk (TurboSFV compat), B2SUMS filename
• Interoperable with GNU b2sum (two-space format, verified in tests)

Design decisions

• BLAKE2b only, no BLAKE2s. Matches b2sum, which is also blake2b-only. If demand arises, blake2s can be added as a separate type later.
• --length in bits, consistent with b2sum -l and the approach adopted for b3. Validated as a multiple of 8 and against blake2b's 8-512 bit range.
• .blk as secondary extension. TurboSFV uses .blk for BLAKE2B files. Kept as auto detect fallback, .b2 is the default for creation.
• Shared HashSumBase. Extracted from the existing BLAKE3 handler. BLAKE2 and BLAKE3 are now thin subclasses with type specific attributes and an _is_valid_filename hook. Test helpers are also shared (_hashsum_roundtrip_test, _hashsum_fixture_test, _hashsum_tool_compat_test, etc.).
• Filename gated auto-detection (same safety rationale as b3). Prevents confusion with sha* types that produce identical length hex on content.

Digest length handling

• Oversized digests in a .b2 file are rejected as a malformed checksum line (cferror, exit 64), not treated as file corruption.
• --length > max_digest_size is validated up-front in make() with a clean --length for b2 must be between 8 and 512 error (exit 1), no traceback.
• --length not a multiple of 8 is still rejected by the shared _parse_hash_length validator.

Changes

• lib/cfv/hash.py — getfileblake2b()
• lib/cfv/common.py — HashSumBase extraction, BLAKE2 class, up-front max_digest_size validation, help text, atrem regex
• test/test.py — shared _hashsum_* test helpers (replace standalone b3 helpers), b2 in fmt_info, full coverage
• test/testdata/ — 7 fixtures: test.b2, test.blk (TurboSFV BLAKE2B-512 reference), B2SUMS, test.blake2b-{256,384}.b2, testcrlf.b2, testcrcrlf.b2
• README.md, cfv.1, Changelog