docs: add cross-layer binding

[felippemsc]

Summary

• Add device_public_key_fingerprint field to JWT payload examples and field tables in sidecar format and cryptographic specs docs
• Add fingerprint verification to cross-validate pseudocode and verification steps
• Add "Cross-Layer Binding" subsection to the two-layer trust model explanation
• Add "Cross-Layer Substitution" threat scenario to the threat model
• Add fingerprint check to the verification checklist

Test plan

• npm run build passes
• Sidecar format page shows device_public_key_fingerprint in JWT payload and fields table
• Cryptographic specs page shows fingerprint check in cross-validate pseudocode
• Two-layer trust page explains cross-layer binding
• Threat model page includes cross-layer substitution threat and updated checklist

[vercel]

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Project	Deployment	Actions	Updated (UTC)
signedshot-docs	Ready	Preview, Comment	Feb 18, 2026 1:30am