Add EC sync and identify endpoints

[ChristianPavilonis]

Summary

• Add GET /sync pixel sync endpoint with required query validation, partner return-domain checks, consent gating (including query fallback decode), redirect reason handling, and rate limiting.
• Add GET /identify plus OPTIONS /identify with publisher-origin CORS handling, consent-first behavior (403 deny / 204 no EC), degraded KV-read behavior, and bidstream-filtered UID/EID response headers.
• Wire new routes in the Fastly adapter, extend EC context helpers, and add/expand unit coverage for sync and identify request/response matrices.

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace
• cd crates/js/lib && npx vitest run
• cd crates/js/lib && npm run format
• cd docs && npm run format

Follow-ups

• /sync currently approximates the story's 1-hour rate limit using a per-minute Fastly counter window due current public API limits; move to exact hour enforcement when platform support is available.