Add ②편 English mirror + figures + KO refinements

- New: _posts/2026-04-21-nccl-algorithms.md (English).
  Mirror of the Korean post. No em dashes, no middle dots.
- Figures (assets/img/posts/nccl-algorithms/): Ring AllReduce
  step-by-step, power-of-two binary tree, NCCL Double Binary Tree,
  Bruck dataflow, PAT 8-rank, Summit BW + Latency scaling.
- KO refinements: §1 contract → 한 줄, §2 cost model 도입 보강,
  §3 의 algorithm 분류를 NCCL 의 실제 enum 7 / pattern 6 / proto 3
  으로 정리, §4 deep-dive 에 figure 7 장 삽입, References 를 ①편
  포맷으로 정리, italic 단일 asterisk 제거.

Author: roycho96