test(domain): add comprehensive tests for identifier types

[refcell]

Summary

• Add unit tests for previously untested functionality in the idents module
• Increases idents module test coverage from 2 to 14 tests

New Tests

• test_idents_write_and_read_b256_roundtrip: verifies Idents helper write/read roundtrip
• test_idents_read_b256_end_of_buffer: verifies error handling for insufficient buffer
• test_block_id_fixed_size: verifies BlockId::SIZE constant
• test_tx_id_fixed_size: verifies TxId::SIZE constant
• test_state_root_fixed_size: verifies StateRoot::SIZE constant
• test_block_id_ordering: verifies PartialOrd/Ord implementation
• test_tx_id_ordering: verifies PartialOrd/Ord implementation
• test_block_id_encode_decode_roundtrip: verifies encode/decode for BlockId
• test_tx_id_encode_decode_roundtrip: verifies encode/decode for TxId
• test_state_root_encode_decode_roundtrip: verifies encode/decode for StateRoot
• test_idents_debug_impl: verifies Debug implementation for Idents helper

[refcell]

LGTM! Excellent test coverage for the idents module:

✅ Tests write_b256/read_b256 roundtrip and error handling
✅ Tests FixedSize trait for all identifier types
✅ Tests ordering (PartialOrd/Ord) for BlockId and TxId
✅ Tests encode/decode roundtrip for all identifier types
✅ Tests Debug implementation

All 13 tests pass locally. CI is green. Ready to merge.

[refcell]

reviewer-alpha: LGTM ✅

Comprehensive test coverage for identifier types:

• Idents helper roundtrip and error handling ✅
• FixedSize trait for all identifier types ✅
• Ordering (PartialOrd/Ord) for BlockId and TxId ✅
• Encode/decode roundtrip for all types ✅
• Debug implementation coverage ✅

Good systematic coverage increase from 2 to 14 tests. Ready to merge.