Add clean README with concise benchmarks

Performance Summary:
- Small volumes (≤32×32): CPU wins (5-10× faster)
- Video workloads (≥112×112): NPU wins (2-3× faster)
- Multi-core scaling: 2-core = 1.3-1.6× over 1-core

Key metrics in scannable table format.
Clear guidance on when to use NPU vs CPU.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jgmelber