04 — Cutover: Remove tokio, axum, tower

Context sources: ./02-event-loop-epoll.md, ./03-hand-rolled-http.md, ../01-problem.md.

Goal

Flip the wo binary off the tokio + axum stack and onto the phase-02 event loop + phase-03 HTTP server. Delete three dependencies from crates/rt/Cargo.toml. REST behaviour visible to reference/rest/blog.rest does not change — same status codes, same response bodies, same endpoint paths.

This is the first phase where the dependency count goes down. Phases 02 and 03 were additive; this one is the switch.

Design decisions (locked)

Atomic swap, single commit. Don't run tokio and the new loop in parallel in production. Flip the binary's main() in one change. Phase 03 already gave us confidence the new stack works end-to-end via http-smoke.
Preserve the Engine trait surface. Arc<Mutex<Engine>> stays exactly as crates/rt/src/engine.rs has it today. The routing layer in crates/rt/src/server.rs — the function that maps service rest blocks to axum MethodRouter — gets rewritten to emit phase-03 Router::route(...) calls instead. Same data flow, different transport.
No tokio — no async. Handlers become synchronous fn(&Request, &Engine) -> Response. The single-threaded event loop already assumes this; removing async fn plumbing simplifies the code. tokio::sync::Mutex becomes std::sync::Mutex (fine in a single-threaded loop since lock contention is impossible).
signalfd replaces tokio::signal::ctrl_c(). Registered as another fd on the loop; reading a SIGINT cleanly exits the loop and closes outstanding connections.
WO_LISTEN env var semantics unchanged. The 127.0.0.1:8080 default + the WO_LISTEN=... override stays exactly as today. Operators don't notice the change.

Scope

Files rewritten inside `crates/rt/`

File	Change	Notes
`src/bin/wo.rs`	Replace `tokio::runtime::Builder::new_current_thread` + `axum::serve` with `EventLoop` + `Listener` + `Router` wiring	The `run()` / `serve()` fns fuse into a single synchronous `run()` that drives the loop
`src/server.rs`	Replace `axum::Router` construction + axum handlers (`async fn list_h(State(st): State<TypeState>) -> impl IntoResponse`) with phase-03 `Router::route(...)` + sync handlers	Handler bodies are otherwise untouched: `engine.lock().list(&ty).map(Json)` logic flows through
`src/engine.rs`	`tokio::sync::Mutex` → `std::sync::Mutex`; `.lock().await` → `.lock().unwrap()`	Only the wrapper changes; row logic intact

`Cargo.toml` delta

 [dependencies]
 anyhow     = "1"
 serde      = { version = "1", features = ["derive"] }
 serde_json = "1"
-tokio      = { version = "1", features = ["rt", "macros", "net", "signal", "sync", "time"] }
-axum       = "0.7"
-tower      = "0.4"
 libc       = "0.2"

Three deps gone. Four remaining: anyhow, serde, serde_json, libc.

Files deleted

None. The phase-02 event/ module and phase-03 http/ module stay in place and now become the primary code path.

Handler signature change

Before (axum + tokio):

async fn list_h(State(st): State<TypeState>) -> impl IntoResponse {
    let eng = st.engine.lock().await;
    match eng.list(&st.ty) {
        Ok(rows) => (StatusCode::OK, Json(json!(rows))).into_response(),
        Err(e)   => (StatusCode::INTERNAL_SERVER_ERROR, e.to_string()).into_response(),
    }
}

After (sync, event loop):

fn list_h(_req: &Request, st: &TypeState) -> Response {
    let eng = st.engine.lock().unwrap();
    match eng.list(&st.ty) {
        Ok(rows) => Response::ok().json(&serde_json::json!(rows)),
        Err(e)   => Response::status(Status::INTERNAL_SERVER_ERROR).body(e.to_string()),
    }
}

Twelve handlers total — one pair per {list, get, create, update, delete} × four types. Mechanical rewrite.

Exit criteria

cargo build at root — compiles with four deps (not seven).
cargo test --lib — all 14 existing rt unit tests still pass. A new test in src/server.rs exercises the router build from a compiled catalog (no HTTP, just static registration).
End-to-end REST smoke — the 20-assertion battery from reference/rest/blog.rest must pass byte-identical to Stage 2 today. Script:
```
WO_LISTEN=127.0.0.1:8765 cargo run --bin wo -- run docs/examples/blog &
# ... curl each block, check expected status
```
Graceful shutdown. SIGINT on the process exits cleanly (no panic, no orphan fds). Validate with strace -f -e signalfd4,close on shutdown.
cd reference/crates && cargo build && cargo test still green.
Dep audit. cargo tree -p rt --depth 1 shows libc as the only non-transitive external dep beyond anyhow, serde, serde_json.

Non-scope

No JSON replacement. serde + serde_json are still imported and used. Phase 05 removes them.
No inotify / sendfile. Stage 3 capabilities. Phases 07 and 08.
No io_uring. The EventLoop keeps using epoll here; swapping is a later phase.
No crate extraction. event/ and http/ stay inside crates/rt/src/. The empty crates/event/ and crates/http/ sibling crates wait for second consumers.

Risk

The .rest files are the safety net — 20 assertions that every Stage 2 endpoint returns the expected status. If one breaks after the cutover, the fix is almost always in the handler rewrite (sync semantics + the new Response::json(...) helper). No transport-layer regression should survive phase 03's http-smoke passing.

Verification

cargo build                                         # 4 deps, no tokio/axum/tower
cargo test --lib                                    # 14 + any new server.rs tests green

# end-to-end
WO_LISTEN=127.0.0.1:8765 cargo run --bin wo -- run docs/examples/blog &
PID=$!
sleep 2
# every block in reference/rest/blog.rest, via curl, checking %{http_code}
# (copy-paste the 20-assertion script from the Stage 2 turn that verified blog.rest)
kill $PID

cd reference/crates && cargo build && cargo test    # v1 untouched

After this phase

Phase 05 removes serde + serde_json by writing a minimal JSON parser + emitter against the new http::Response::json() surface. At the end of phase 06, crates/rt/Cargo.toml is down to libc alone — the stated end goal.

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