feat: add initial

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

todos/SQ-012-consumer-groups.md

@@ -0,0 +1,40 @@
+# 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