Make configuration values injectable rather than hardcoded

Add config structs with sensible defaults for tunable parameters:
- JetStreamConfig for stream retention (1 year) and replica count (1)
- HashRingConfig for virtual nodes per physical node (150)
- ShardConfig for shard count (1024) and replication factor (1)

Each component gets a new WithConfig constructor that accepts custom
configuration, while the original constructors continue to work with
defaults. Zero values in configs fall back to defaults for backward
compatibility.

Closes #38

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cluster/distributed.go

@@ -200,11 +200,11 @@ func (dvm *DistributedVM) GetClusterInfo() map[string]interface{} {
 	nodes := dvm.cluster.GetNodes()
 
 	return map[string]interface{}{
-		"nodeId":     dvm.nodeID,
-		"isLeader":   dvm.cluster.IsLeader(),
-		"leader":     dvm.cluster.GetLeader(),
-		"nodeCount":  len(nodes),
-		"nodes":      nodes,
+		"nodeId":    dvm.nodeID,
+		"isLeader":  dvm.cluster.IsLeader(),
+		"leader":    dvm.cluster.GetLeader(),
+		"nodeCount": len(nodes),
+		"nodes":     nodes,
 	}
 }
 
@@ -218,4 +218,4 @@ func (dvr *DistributedVMRegistry) GetActiveVMs() map[string]interface{} {
 // GetShard returns the shard number for the given actor ID
 func (dvr *DistributedVMRegistry) GetShard(actorID string) int {
 	return dvr.sharding.GetShard(actorID)
-}
+}