Reframe docs around fingerprint-first architecture

[nahiyankhan]

Stacked PR 1/4.

Summary

• Establish Ghost as a fingerprint-first architecture in the top-level docs.
• Add the settled architecture memo that defines fingerprint as the durable product-experience contract.
• Reframe older idea docs through the new fingerprint-first hierarchy.

Verification

• Covered by the stack pre-push hook: pnpm build, pnpm check, pnpm test.