feat: add initial

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

todos/SQ-022-simulation-tests.md

@@ -0,0 +1,49 @@
+# SQ-022: Multi-Node Simulation Tests
+
+**Status:** `[ ] TODO`
+**Blocked by:** SQ-021, SQ-019
+**Priority:** High
+
+## Description
+
+Full TigerBeetle-inspired simulation test suite. Spin up multiple nodes with virtual I/O, inject faults, verify invariants.
+
+## Files to Create/Modify
+
+- `crates/sq-sim/src/runtime.rs` - test harness for multi-node simulation
+- `crates/sq-sim/tests/invariants.rs` - invariant checker functions
+- `crates/sq-sim/tests/scenarios/mod.rs`
+- `crates/sq-sim/tests/scenarios/single_node.rs` - S01-S04
+- `crates/sq-sim/tests/scenarios/multi_node.rs` - S05-S08
+- `crates/sq-sim/tests/scenarios/failures.rs` - S09-S12
+
+## Scenarios
+
+- **S01:** Single node, single producer, single consumer - baseline
+- **S02:** Single node, concurrent producers - offset ordering
+- **S03:** Single node, disk full during write - graceful error
+- **S04:** Single node, crash and restart - WAL recovery
+- **S05:** Three nodes, normal operation - replication works
+- **S06:** Three nodes, one crashes - remaining two continue
+- **S07:** Three nodes, network partition (2+1) - majority continues
+- **S08:** Three nodes, S3 outage - local WAL accumulates
+- **S09:** Consumer group, offset preservation
+- **S10:** High throughput burst - no message loss
+- **S11:** Slow consumer with WAL trimming - falls back to S3
+- **S12:** Node rejoins after long absence - catches up
+
+## Invariants (checked after every step)
+
+1. No acked message is ever lost
+2. Offsets strictly monotonic, no gaps
+3. CRC integrity on all reads
+4. Consumer group offsets never regress
+5. After network heal, replicas converge
+6. WAL never trimmed before S3 confirmation
+
+## Acceptance Criteria
+
+- [ ] All 12 scenarios pass
+- [ ] Each scenario runs with multiple random seeds (at least 10)
+- [ ] Invariant violations produce clear diagnostic output
+- [ ] Tests complete in < 60 seconds total