Fastly entry point switch (dual-path with flag)

[aram356]

Description

Add feature-flagged dual-path entry point: edgezero_enabled config flag with unwrap_or(false) safe default. EdgeZero path dispatches through TrustedServerApp::build_app(); legacy path preserved as legacy_main().

Done when

• Both paths compile
• Legacy cleanup issue filed
• Per-PR gates pass

Blocked by

PR 13

References

• docs/internal/EDGEZERO_MIGRATION.md — Phase 3, PR 14
• Part of EdgeZero migration #480