# SQ-007: Storage Engine Facade

**Status:** `[ ] TODO`
**Blocked by:** SQ-006
**Priority:** High

## Description

Unified read/write interface wrapping WAL writer + reader + offset index. This is the single entry point for all storage operations used by the server.

## Files to Create/Modify

- `crates/sq-storage/src/engine.rs` - StorageEngine

## StorageEngine API

```rust
pub struct StorageEngine<F: FileSystem, C: Clock> {
    fs: Arc<F>,
    clock: Arc<C>,
    config: WalConfig,
    writers: HashMap<(String, u32), WalWriter<F>>,
    index: OffsetIndex,
}

impl<F: FileSystem, C: Clock> StorageEngine<F, C> {
    pub async fn new(fs: Arc<F>, clock: Arc<C>, config: WalConfig) -> Result<Self>;
    pub async fn append(&mut self, topic: &str, partition: u32, key: Option<&[u8]>, value: &[u8], headers: &[Header]) -> Result<u64>;
    pub async fn read(&self, topic: &str, partition: u32, from_offset: u64, limit: usize) -> Result<Vec<Message>>;
    pub async fn recover(&mut self) -> Result<()>;  // Rebuild state from existing WAL files on startup
    pub fn closed_segments(&self) -> Vec<ClosedSegment>;  // For S3 shipper
}
```

## Acceptance Criteria

- [ ] Write 1000 messages, read from offset 0, verify all present
- [ ] Write, read from offset 500, verify correct slice returned
- [ ] Read with limit, verify at most N messages returned
- [ ] Write to multiple topics/partitions, verify isolation (no cross-contamination)
- [ ] Recovery: write messages, drop engine, create new engine, call recover(), read all messages back
- [ ] Segment rotation happens transparently during append