[Phase 1a] Security review spike for snapshot integrity and TTL

[usepowershell]

Part of #98

Security review of stateless snapshot format and validation.

Subtasks

• Review HMAC-SHA256 signature approach
• Confirm no sensitive secrets in snapshot payload
• Validate 5-minute TTL behavior
• Define key management/rotation approach
• Threat model for tampering/replay
• Produce go/no-go recommendation

Effort: 1 week