v1.1.5 - Fix CRITICAL segfault: Use wrapper nodes for no-kind-filter subscriptions

The kind index optimization in v1.1.4 introduced a critical bug that caused
segmentation faults in production. The bug was in add_subscription_to_kind_index()
which directly assigned sub->next for no-kind-filter subscriptions, corrupting
the main active_subscriptions linked list.

Root Cause:
- subscription_t has only ONE 'next' pointer used by active_subscriptions list
- Code tried to reuse 'next' for no_kind_filter_subs list
- This overwrote the active_subscriptions linkage, breaking list traversal
- Result: segfaults when iterating subscriptions

Fix:
- Added no_kind_filter_node_t wrapper structure (like kind_subscription_node_t)
- Changed no_kind_filter_subs from subscription_t* to no_kind_filter_node_t*
- Updated add/remove functions to use wrapper nodes
- Updated broadcast function to iterate through wrapper nodes

This follows the same pattern already used for kind_index entries and
prevents any corruption of the subscription structure's next pointer.

src/subscriptions.h

@@ -69,6 +69,12 @@ typedef struct kind_subscription_node {
     struct kind_subscription_node* next;       // Next subscription for this kind
 } kind_subscription_node_t;
 
+// No-kind-filter list entry - wrapper to avoid corrupting subscription->next pointer
+typedef struct no_kind_filter_node {
+    subscription_t* subscription;              // Pointer to subscription
+    struct no_kind_filter_node* next;          // Next subscription in no-kind list
+} no_kind_filter_node_t;
+
 // Per-IP connection tracking
 typedef struct ip_connection_info {
     char ip_address[CLIENT_IP_MAX_LENGTH];  // IP address
@@ -87,7 +93,7 @@ struct subscription_manager {
 
     // Kind-based index for fast subscription lookup (10x performance improvement)
     kind_subscription_node_t* kind_index[65536];  // Array of subscription lists, one per kind
-    subscription_t* no_kind_filter_subs;          // Subscriptions with no kind filter (must check all events)
+    no_kind_filter_node_t* no_kind_filter_subs;   // Subscriptions with no kind filter (wrapper nodes)
 
     // Configuration
     int max_subscriptions_per_client;       // Default: 20