Phase 2, Task 6: Migrate entry point from #[fastly::main] to raw main()

[aram356]

Parent: #563
Phase: #567

Summary

Migrate main.rs from #[fastly::main] to an undecorated main() with Request::from_client(). Required because stream_to_client() / send_to_client() are incompatible with #[fastly::main]'s return-based model.

Changes

• Replace #[fastly::main] fn main(req: Request) -> Result<Response, Error> with undecorated fn main()
• Use Request::from_client() to get incoming request
• Explicit error handling via to_error_response().send_to_client()
• All non-streaming routes use resp.send_to_client()
• No fastly::init() needed (doesn't exist in SDK 0.11.12)

File: crates/trusted-server-adapter-fastly/src/main.rs:32-68

Plan

See docs/superpowers/plans/2026-03-25-streaming-response.md — Task 6