docs: sharpen architecture history arrowhead

[forhappy]

Summary

• Add a dedicated, fuller triangular marker for the curved history connector.
• Keep the straight connector arrowheads unchanged.
• Keep the README’s existing SVG reference and diagram content intact.

Validation

• Rendered the SVG at README scale with rsvg-convert.
• git diff --check passed.
• Branch is based directly on origin/main.