Utility layer type migration + compat adapter

[aram356]

Description

Migrate utility modules (cookies, GDPR, HTTP helpers, synthetic) from fastly::Request/Response to http::Request/Response. Create compat.rs compatibility adapter to bridge remaining Fastly helper APIs.

Done when

• Utilities use http types
• compat.rs created
• Per-PR gates pass

Blocked by

PRs 3-10

References

• docs/internal/EDGEZERO_MIGRATION.md — Phase 2, PR 11
• Part of EdgeZero migration #480