Complete incomplete Viterbi lattice tests

[eunseo9311]

The test_viterbi test in lattice.rs was marked XXX: incomplete —
it only checked .len() without verifying node IDs or token content.

This PR completes it and adds three new test cases:

• test_viterbi: verify node IDs and tokens returned by viterbi(),
  and confirm score-based path selection picks the higher-score path
• test_viterbi_multibyte: lattice over a multi-byte UTF-8 string
  ("テスト"), including a multi-character spanning node
• test_viterbi_empty: empty string edge case
• test_sample_smoke: structural validity check for sample() —
  sampled tokens must always concatenate back to the original input