# SQ-012: Consumer Groups & Offset Tracking

**Status:** `[ ] TODO`
**Blocked by:** SQ-011
**Priority:** Medium

## Description

Consumer group offset management: store committed offsets, use them as default start position for Subscribe, and implement the Ack RPC.

## Files to Create/Modify

- `crates/sq-storage/src/consumer_offsets.rs` - in-memory map with file persistence
- `crates/sq-server/src/grpc/data_plane.rs` - Ack RPC impl; update Subscribe to use committed offset
- `crates/sq-storage/src/engine.rs` - add consumer offset methods

## ConsumerOffsets API

```rust
pub struct ConsumerOffsets {
    offsets: HashMap<(String, String, u32), u64>,  // (group, topic, partition) -> offset
    persist_path: PathBuf,
}

impl ConsumerOffsets {
    pub fn commit(&mut self, group: &str, topic: &str, partition: u32, offset: u64) -> Result<()>;
    pub fn get_committed(&self, group: &str, topic: &str, partition: u32) -> Option<u64>;
    pub async fn persist(&self) -> Result<()>;
    pub async fn load(path: &Path) -> Result<Self>;
}
```

## Acceptance Criteria

- [ ] Commit offset, query it back, verify correct
- [ ] Commit offset, persist to file, load from file, verify preserved
- [ ] Ack RPC: commit offset via gRPC, verify stored
- [ ] Subscribe without start_offset uses committed offset for the consumer group
- [ ] Subscribe with explicit start_offset overrides committed offset
- [ ] Two consumers in same group: both see same committed offset