[TekenujUi90]

Summary

• Add an ADR template, index, and three accepted records for route rendering, shared state, and backend/Soroban boundaries.
• Link the records from the README and point each decision to its detailed architecture guide.

Closes #330

Validation

• git diff --check passed.
• Checked local Markdown links in the ADR index, records, and README.
• Runtime checks were not needed for documentation-only changes. npm ci --ignore-scripts could not install because the checked-in package lock is out of sync with package.json (missing and mismatched package entries). No dependency files were changed.

[vercel]

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