feat: implement cross-node event broadcasting with NATSEventBus (#151)

This PR implements cross-node event broadcasting for aether.

Changes:
- UpdateVersionCache method in JetStreamEventStore
- SubscribeToEventStored helper in NATSEventBus
- Integration tests for cross-node scenarios
- Example code demonstrating NATSEventBus + JetStreamEventStore

Tests: All integration tests passing.
Co-authored-by: Claude Code <noreply@anthropic.com>
Co-authored-by: Hugo Nijhuis <hugo.nijhuis@flowmade.one>
Reviewed-on: #151

.product-strategy/ISSUE_VM_RUNTIME.md

@@ -0,0 +1,100 @@
+# Issue: Implement VM/Runtime for Actors
+
+## Problem
+
+Only interfaces exist for `Runtime` and `VirtualMachine` in `cluster/types.go` and `cluster/distributed.go`, but no actual implementation. Actors cannot be created, started, stopped, or have their state managed.
+
+## Required Components
+
+### 1. VM Implementation (cluster/vm.go - new)
+```go
+type VirtualMachine struct {
+    actorID   string
+    eventStore aether.EventStore
+    state     map[string]interface{}
+    version   int64
+}
+```
+
+Methods needed:
+- `GetID()`, `GetActorID()`, `GetState()` - already in interface
+- `Start()` - replay events to rebuild state
+- `ProcessEvent(event *aether.Event)` - apply event to state
+- `Stop()` - persist final state
+- `GetVersion()` - current event version
+
+### 2. Runtime Implementation (cluster/runtime.go - new)
+```go
+type Runtime struct {
+    natsConn   *nats.Conn
+    eventStore aether.EventStore
+    vmRegistry VMRegistry // map[actorID]*VirtualMachine
+    config     RuntimeConfig
+}
+```
+
+Methods needed:
+- `Start()` - initialize and start processing
+- `LoadModel(model eventstorming.Model)` - register domain types
+- `SendMessage(message RuntimeMessage)` - route to appropriate VM
+- `GetActiveVMs()` - return map of active VMs
+- `CreateVM(actorID string)` - create new VM instance
+- `StopVM(actorID string)` - persist and stop VM
+
+### 3. Event Processing
+- Subscribe to actor's event stream
+- Replay events to build initial state
+- Apply new events as they arrive
+- Handle event versions and conflicts
+
+## Suggested Design
+
+### VM Lifecycle
+```
+1. Actor message arrives for actor-123
+2. Runtime checks if VM exists for actor-123
+3. If not, create VM:
+   - Replay events from event store
+   - Rebuild state
+4. Route message to VM
+5. VM processes message -> creates new events
+6. Events persisted to event store
+7. VM state updated
+```
+
+### State Management
+- State derived from event replay
+- No separate state store needed
+- Can snapshot periodically for performance
+- Version conflict handling using existing EventStore
+
+## Implementation Steps
+
+1. **Create VM struct** in `cluster/vm.go`
+2. **Implement event replay** to rebuild state
+3. **Create Runtime** in `cluster/runtime.go`
+4. **Register Runtime with cluster** via `SetVMProvider`
+5. **Implement message processing** - validate against model
+6. **Add version conflict handling** using existing EventStore
+7. **Write tests** - mock event store, test state transitions
+
+## File Structure
+
+```
+cluster/
+├── vm.go              # VirtualMachine implementation
+├── runtime.go         # Runtime implementation
+├── vm_test.go         # VM tests
+├── runtime_test.go    # Runtime tests
+└── integration_test.go # Integration tests
+```
+
+## Acceptance Criteria
+
+- [ ] VM can be created with actor ID
+- [ ] VM replays events to build state
+- [ ] VM processes events and updates state
+- [ ] VM persists current version
+- [ ] Runtime can create/stop VMs
+- [ ] Runtime manages VM registry
+- [ ] Integration test with NATS and JetStream