feat(u4): add nibble pair byte adapter

[brenorb]

Summary

• add u4_nibbles_to_byte to pack a high/low nibble pair into one byte
• provide checked and unchecked forms with explicit stack contracts
• reject negative and 16-valued inputs in checked mode
• record script, witness, stack, and static-opcode metrics

Validation

• cargo test --locked arithmetic::u4::bits::tests::exhaustive_nibble_pairs_pack_to_bytes
• cargo test --locked arithmetic::u4::bits::tests::checked_nibble_pair_rejects_malformed_inputs
• cargo test --locked --test primitive_metrics
• python3 tools/kb.py validate
• cargo fmt --check
• git diff --check

The full repository test suite was intentionally not run; the focused primitive and metric checks pass. Dynamic opcode counting is unavailable in the local executor, so the documentation reports static non-push counts.