Security hardening

[rcholic]

1. 5.1 Merkle Hash Chain in Sidecar (0.5 day) ✅

   • Add chain_hash and event_id to ProofEvent
   • Compute SHA-256 chain on record
   • Expose /ledger/chain-head endpoint for verification
   • Expose /ledger/verify endpoint for integrity checks

2. 5.2 Policy Signature Verification (1 day) ✅

   • Add ed25519-dalek dependency
   • Create policy_signer module with Ed25519 signing/verification
   • Add SignedPolicy struct and parsing functions
   • Integrate signature verification into policy_loader
   • Optional enterprise feature (disabled by default)

3. 5.3 Secret Zeroization (0.5 day) ✅

   • Add zeroize dependency
   • Create SecretKeyBytes wrapper with ZeroizeOnDrop
   • Apply to SigningKeyMaterial in mandate module

4. 5.4 SSRF Protection (0.25 day) ✅

   • Create ssrf module with SsrfProtection struct
   • Block private IPs (RFC 1918), localhost, link-local
   • Block cloud metadata endpoints (AWS, GCP, Azure, K8s)
   • Block internal DNS suffixes (.internal, .local, etc.)
   • Integrate into PolicyEngine as implicit deny rule

5. 5.5 Loop Guard (0.5 day) ✅

   • Add lru crate for cache
   • Create loop_guard module with LoopGuard struct
   • Track failures per (principal, action, resource) tuple
   • Configurable thresholds and time windows