docs: use elbow history connector

[forhappy]

Summary

• Replace the curved history connector with a direct horizontal segment and one diagonal corner.
• Rotate the explicit endpoint arrowhead to follow the diagonal segment.
• Keep the cache-busted README diagram reference from PR docs: make history arrowhead explicit #13.

Validation

• Rendered the SVG at README scale with rsvg-convert.
• git diff --check passed.