Follow-up: use pYIN block transition structure for fast Viterbi

[ssmall256]

Summary

Follow-up to #11.

This PR switches the viterbi_impl="fast" pYIN path from building sparse predecessors by scanning a dense transition matrix to building them directly from the pYIN block transition model.

Dependency

• Depends on Add an exact structured pYIN Viterbi backend #11 (pr/pyin-structured-viterbi) being merged first.
• This branch intentionally stacks on top of that branch.

What Changed

• Added compute_transition_structure_pyin_block(...) in libf0/pyin.py
• Fast path now uses this structured builder directly
• Added/updated tests to verify structure and decode parity against dense-matrix-derived structure
• Updated benchmark harness to use the structured builder

Validation

• python -m pytest tests/test_algorithms.py -q (10 passed)
• python -m py_compile libf0/pyin.py tests/test_algorithms.py scripts/benchmark_pyin_viterbi.py
• PYTHONPATH=/Users/sam/Code/libf0 python scripts/benchmark_pyin_viterbi.py --warmup 1 --iters 2 --json
  • decoder parity: true
  • pyin parity (f0/time/conf): true