EdgeZero migration Phase 5 — Delete legacy Fastly path (gated)

[aram356]

Part of #840. Spec: docs/superpowers/specs/2026-07-02-edgezero-full-migration-design.md §5 Phase 5. Gated on 100% EdgeZero rollout (#495).

Delete legacy_main/route_request, compat.rs, and the edgezero_enabled/edgezero_rollout_pct flag machinery. main() calls the EdgeZero path unconditionally.
Acceptance: single EdgeZero entry path on Fastly; no rollout flags; full CI gate green.