EC middleware integration

[aram356]

Parent epic

#532

Description

Wire EcContext into the request pipeline following the two-phase model. EcContext::read_from_request() runs pre-routing like GeoInfo; generate_if_needed() runs inside organic handlers only.

Scope: main.rs, publisher.rs, endpoints.rs, registry.rs

Acceptance criteria

• EcContext::read_from_request() called before route match on every request, passed existing geo_info.
• EC route handlers (/identify, /auction, /api/v1/sync, /admin/*) receive ec_context read-only — never call generate_if_needed(), never mutate.
• Exception: /sync receives &mut ec_context. May assign fallback consent into ec_context.consent when query-param fallback applies.
• handle_publisher_request() and integration_registry.handle_proxy() call ec_context.generate_if_needed(settings, &kv) (best-effort, never 500s).
• ec_finalize_response() runs on every route: withdrawal (cookie delete + tombstone), last_seen (debounced), mismatch reconciliation, new-EC cookie write.
• route_request() return type changes to Result<(), Error>; response flushed via send_to_client(). #[fastly::main] entrypoint changes to match. Pre-routing errors handled via to_error_response() + send_to_client().
• handle_auction() signature changes to accept &KvIdentityGraph and &EcContext.
• Handler refactoring: internal build_consent_context() calls replaced with ec_context.consent. Identity from ec_context.ec_value.
• cargo test --workspace passes with no regressions.

Spec ref

docs/internal/ssc_technical_spec.md §5, §17