feat: add initial

Signed-off-by: kjuulh <contact@kjuulh.io>

todos/SQ-007-storage-engine-facade.md

@@ -0,0 +1,42 @@
+# 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