# SQ-001: Domain Types

**Status:** `[x] DONE`
**Blocked by:** SQ-000
**Priority:** High

## Description

Define the core domain types in `sq-models`: Message, Header, TopicName, Offset, and configuration types. These are the foundational types used throughout the entire system.

## Files to Create/Modify

- `crates/sq-models/src/lib.rs` - re-exports
- `crates/sq-models/src/message.rs` - Message, Header, TopicName, Offset types
- `crates/sq-models/src/config.rs` - TopicConfig, WalConfig

## Key Types

```rust
pub struct Message {
    pub offset: u64,
    pub topic: TopicName,
    pub key: Option<Vec<u8>>,
    pub value: Vec<u8>,
    pub headers: Vec<Header>,
    pub timestamp_ms: u64,
}

pub struct Header {
    pub key: String,
    pub value: Vec<u8>,
}

pub struct TopicName(pub String);

pub struct WalConfig {
    pub max_segment_bytes: u64,    // default 64MB
    pub max_segment_age_secs: u64, // default 60s
    pub data_dir: PathBuf,
}

pub struct TopicConfig {
    pub name: TopicName,
    pub partitions: u32,           // default 1
    pub replication_factor: u32,   // default 3
}
```

## Acceptance Criteria

- [ ] All types compile and are public
- [ ] `Message` implements `Clone`, `Debug`, `PartialEq`
- [ ] Unit test: construct a Message, verify all fields
- [ ] Property test (optional): arbitrary Message roundtrip through Debug/PartialEq