# SQ-005: WAL Segment Reader

**Status:** `[x] DONE`
**Blocked by:** SQ-004
**Priority:** High

## Description

Implement the WAL segment reader that reads messages from segment files, supporting seek-to-offset and forward scanning.

## Files to Create/Modify

- `crates/sq-storage/src/wal/reader.rs` - WalReader with open, read_from, iterator

## WalReader API

```rust
pub struct WalReader<F: FileSystem> {
    fs: Arc<F>,
}

impl<F: FileSystem> WalReader<F> {
    pub fn new(fs: Arc<F>) -> Self;
    pub async fn read_segment(&self, path: &Path) -> Result<Vec<Message>>;
    pub async fn read_from_offset(&self, path: &Path, offset: u64) -> Result<Vec<Message>>;
    pub async fn read_segment_header(&self, path: &Path) -> Result<SegmentHeader>;
}
```

## Acceptance Criteria (using InMemoryFileSystem)

- [ ] Write N messages with writer, read all back with reader, verify equality
- [ ] Read from a specific offset in the middle of a segment
- [ ] Corrupted record mid-segment: reader returns error for that record, can report partial results
- [ ] Empty segment (header only): reader yields zero messages
- [ ] Invalid magic bytes: reader returns descriptive error
- [ ] Truncated record at end of segment (partial write): reader stops cleanly at last complete record