Implement position-aware density evolution for SC-LDPC codes:
- sc_density_evolution(): flooding-schedule DE tracking per-position
error rates, demonstrating the wave decoding effect
- compute_sc_threshold(): binary search for SC-LDPC threshold
Uses flooding schedule (not layered) to avoid belief divergence
from cross-position message interference in the coupled chain.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>