# SQ-019: Virtual Network for Simulation

**Status:** `[x] DONE`
**Blocked by:** SQ-003
**Priority:** Medium

## Description

Virtual network layer for multi-node simulation testing. Enables partition, latency, and packet drop injection.

## Files to Create/Modify

- `crates/sq-sim/src/network.rs` - VirtualNetwork with message queues and fault injection

## VirtualNetwork API

```rust
pub struct VirtualNetwork {
    queues: HashMap<(NodeId, NodeId), VecDeque<Vec<u8>>>,
    partitions: HashSet<(NodeId, NodeId)>,
    latency: Option<(Duration, Duration)>,  // min, max
    drop_probability: f64,
}

impl VirtualNetwork {
    pub fn new() -> Self;
    pub fn partition(&mut self, a: NodeId, b: NodeId);
    pub fn heal(&mut self, a: NodeId, b: NodeId);
    pub fn heal_all(&mut self);
    pub fn set_latency(&mut self, min: Duration, max: Duration);
    pub fn set_drop_probability(&mut self, prob: f64);
    pub async fn send(&self, from: NodeId, to: NodeId, msg: Vec<u8>) -> Result<()>;
    pub async fn recv(&self, node: NodeId) -> Result<(NodeId, Vec<u8>)>;
    pub fn deliver_pending(&mut self);  // Process queued messages
}
```

## Acceptance Criteria

- [ ] Two virtual nodes exchange messages successfully
- [ ] Partition: messages from A to B are dropped
- [ ] Heal: messages resume flowing after heal
- [ ] Latency injection: messages are delayed
- [ ] Drop probability: some messages are randomly dropped
- [ ] Bidirectional partition: neither direction works