docs: replace evidence ledger stack map

[solomonneas]

Summary

• Replace the evidence ledger README's Mermaid stack map with a generated 960 by 540 SVG.
• Group the flow into sources, the adapter contract, the local ledger, and reader surfaces.
• Keep the JSON workflow source beside the SVG so later edits remain reproducible.

Why

The old Mermaid map fed 12 source nodes into the center of one diagram. On GitHub, its edges crossed labels and the map needed pan and zoom controls. The replacement keeps the crawler coverage and ownership boundary while limiting each stage to three cards.

Verification

• ./scripts/verify
• plating workflow docs/assets/workflows/stack-map.json --out docs/assets/stack-map.svg
• Inspected desktop and 390 px responsive renders

[coderabbitai]

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✨ Finishing Touches

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• Commit unit tests in branch fire/evidence-ledger-stack-map-20260725

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