docs: sharpen architecture history arrowhead

[forhappy]

Summary

• Add a dedicated, fuller triangular marker for the curved history connector.
• Keep the straight connector arrowheads unchanged.
• Preserve the README SVG source and remove no text/content.

Validation

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

[forhappy]

Superseded by the clean follow-up #12, which is based directly on main and contains only the arrowhead marker change.