# SQ-004: WAL Segment Writer

**Status:** `[x] DONE`
**Blocked by:** SQ-002, SQ-003
**Priority:** High

## Description

Implement the WAL segment writer that appends records to segment files with fsync for durability. Handles segment rotation when size or time thresholds are exceeded.

## Files to Create/Modify

- `crates/sq-storage/src/wal/writer.rs` - WalWriter with append + fsync + rotation
- `crates/sq-storage/src/wal/segment.rs` - segment header encoding/decoding

## Segment Header Format (32 bytes)

```
[magic: [u8; 4]]    = b"SQWL"
[version: u16]       = 1
[topic_len: u16]
[topic: [u8; 20]]    (padded/truncated)
[partition: u32]
```

## WalWriter API

```rust
pub struct WalWriter<F: FileSystem> {
    fs: Arc<F>,
    config: WalConfig,
    topic: TopicName,
    partition: u32,
    active_segment: Option<Box<dyn FileHandle>>,
    segment_base_offset: u64,
    segment_position: u64,
    next_offset: u64,
    segment_opened_at: Instant,
}

impl<F: FileSystem> WalWriter<F> {
    pub async fn new(fs: Arc<F>, config: WalConfig, topic: TopicName, partition: u32) -> Result<Self>;
    pub async fn append(&mut self, key: Option<&[u8]>, value: &[u8], headers: &[Header], timestamp_ms: u64) -> Result<u64>;  // returns offset
    pub async fn close_active_segment(&mut self) -> Result<Option<ClosedSegment>>;
    pub fn next_offset(&self) -> u64;
}
```

## Acceptance Criteria (using InMemoryFileSystem)

- [ ] Write 1 message, verify segment file exists with correct header + record
- [ ] Write 100 messages, verify all offsets are monotonically increasing (0, 1, 2, ...)
- [ ] Segment rotation: write until size > max_segment_bytes, verify new segment created
- [ ] Segment rotation: advance clock past max_segment_age, verify rotation on next write
- [ ] fsync failure: set fault on InMemoryFS, verify append() returns error
- [ ] fsync failure: offset is NOT advanced (can retry the write)
- [ ] Segment directory structure: `{data_dir}/{topic}/{partition}/{base_offset}.wal`

## Notes

- sq-storage depends on sq-sim for the FileSystem trait
- Writer must call fsync after every append (or batch of appends)
- `ClosedSegment` contains the path and offset range of the completed segment