docs: replace Mermaid flow with reliable SVG

[AetherAI3]

What changed

• Replaced the fragile Mermaid block with a committed, accessible SVG architecture flow.
• Embedded the SVG directly in the README, keeping the Nano/host boundary easy to scan without invoking a Mermaid renderer.

Root cause

The README visual depended on Mermaid parsing in the display surface. Rather than continue to patch parser-sensitive syntax, this follow-up uses a static vector asset that renders as a normal GitHub image.

Impact

Readers get a polished architecture visual on every GitHub surface, while the source stays versioned and reviewable.

Validation

• Parsed the SVG as XML.
• Checked the README-to-asset path.
• Ran git diff --check.

This is a stacked follow-up to #2 and targets codex/readme-community-followup so it contains only the rendering repair.