# Task 0 — Backend injection seam + tempfile dev-dep

> **Index:** [README](README.md). **Spec:** [design](../../specs/2026-04-29-quantum-tutor-design.md).

## Goal

Make `QuantumBridgeServer` hold `Arc<dyn Backend>` so the upcoming tutor tools depend on `&dyn Backend` (spec §3, CLAUDE.md §3). Existing v1 tools (`list_backends`, `validate_circuit`, `run_circuit`) are **not** refactored — they continue to construct their own `LocalSimulator`. Only the new path complies with DIP. Also add `tempfile` for sandboxed tests in later tasks.

## Prerequisites

None. This is the starting point.

## Files

- Modify: `Cargo.toml`
- Modify: `src/tools/mod.rs`
- Modify: `src/main.rs`

## Steps

- [ ] **Step 1: Add `tempfile` to `[dev-dependencies]` in `Cargo.toml`**

```toml
tempfile = "3"
```

- [ ] **Step 2: Replace the unit struct in `src/tools/mod.rs`**

```rust
use std::sync::Arc;
use crate::executor::{Backend, LocalSimulator};

#[derive(Clone)]
pub struct QuantumBridgeServer {
    pub backend: Arc<dyn Backend>,
}

impl QuantumBridgeServer {
    pub fn new(backend: Arc<dyn Backend>) -> Self {
        Self { backend }
    }

    pub fn with_local_simulator() -> Self {
        Self::new(Arc::new(LocalSimulator::new()))
    }
}
```

The existing tool methods inside `#[tool_router(server_handler)] impl QuantumBridgeServer { ... }` are not modified.

- [ ] **Step 3: Update the construction in `src/main.rs`**

```rust
let service = QuantumBridgeServer::with_local_simulator()
    .serve(stdio())
    .await
    .inspect_err(|e| tracing::error!("serving error: {:?}", e))?;
```

- [ ] **Step 4: Verify the existing test suite still passes**

```bash
cargo build && cargo test --lib 2>&1 | grep -E "test result|FAILED"
```

Expected: every existing suite green; the change is purely additive.

- [ ] **Step 5: Commit**

```bash
git add Cargo.toml src/tools/mod.rs src/main.rs
git commit -m "refactor: QuantumBridgeServer holds Arc<dyn Backend>; add tempfile dev-dep"
```