docs: add coroutine start behavior comparison, enlarge risk quadrant chart

- Add Section 4.3 comparing eager (Boost) vs lazy (C++20) coroutine
  start behavior, clarifying the initial-yield pattern was a Boost API
  design choice not a rippled requirement
- Increase risk quadrant chart dimensions (800x800) with larger fonts
  and spread out data points to prevent text overlap
- Center the quadrant chart
- Renumber subsections 4.4-4.8 to accommodate new section

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

Author: pratikmankawde