v0.3.1: per-subscriber monitor thread — fix bitmap leak

Bug: handshake_subscriber assigned bit + activated slot но НЕ tracked
client_fd. Когда subscriber container exited, socket closed on client side
но producer не detected → bit оставался set forever → после 32 connections
subscribe_create('cam-X'): too many subscribers (max 32).

Симптом в production: каждый pipeline recreate accumulated 1 stale subscriber.
После 4-5 recreate операций publishers перестали accept new pipeline →
"too many subscribers" crash loop.

Fix: после успешного handshake spawn detached pthread monitoring socket
via blocking recv(). recv() returns 0 (EOF) когда other side closes —
monitor clears bit (subscriber_bitmap &= ~(1<<bit)) + state[bit] = 0,
closes fd, exits.

Cost: 1 thread per active subscriber. Max 32 threads — небольшой
overhead. Threads detached, no join needed.

Stress test: 5x pipeline recreate без single "too many subscribers" error.
Раньше: 2-3 recreate → bitmap overflow.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

libcuframes/src/producer.c

@@ -628,6 +628,38 @@ int cuframes_publisher_publish_packet(cuframes_publisher_t *pub,
 
 /* ─── Accept thread + handshake ──────────────────────────────────────── */
 
+/* Per-subscriber lifecycle monitor — detects socket close (subscriber container
+ * exited / crashed) и освобождает bit + subscribers[] slot. Без этого каждый
+ * pipeline recreate leaks bit → bitmap overflows after 32 connections. */
+struct sub_monitor_args {
+    struct cuframes_publisher *pub;
+    int fd;
+    uint32_t bit;
+};
+
+static void *subscriber_monitor_thread(void *arg) {
+    struct sub_monitor_args *m = (struct sub_monitor_args *)arg;
+    char buf[64];
+    /* Blocking read — return 0 (EOF) когда other side close socket, или
+     * <0 on error. Любой control message (PING — TODO в будущем) just consumed. */
+    while (1) {
+        ssize_t n = recv(m->fd, buf, sizeof(buf), 0);
+        if (n <= 0) {
+            /* Subscriber dead — clear bit + slot state. */
+            atomic_fetch_and_explicit(&m->pub->hdr->subscriber_bitmap,
+                ~(1ULL << m->bit), memory_order_release);
+            atomic_store_explicit(&m->pub->hdr->subscribers[m->bit].state, 0,
+                memory_order_release);
+            close(m->fd);
+            CUFRAMES_LOG_INFO("subscriber bit=%u disconnected — freed",
+                              m->bit);
+            free(m);
+            return NULL;
+        }
+        /* future: parse control msgs (PING, UNSUBSCRIBE) here */
+    }
+}
+
 static void *accept_thread_main(void *arg) {
     struct cuframes_publisher *pub = (struct cuframes_publisher *)arg;
     while (!pub->stop_flag) {
@@ -640,14 +672,12 @@ static void *accept_thread_main(void *arg) {
             CUFRAMES_LOG_WARN("accept: %s", strerror(errno));
             continue;
         }
-        /* Synchronous handshake — после ответа socket остаётся открытым для
-         * lifetime signals (SHUTDOWN, PING). Close на error. */
+        /* Handshake — на error close socket (no monitor spawned). На success
+         * monitor thread становится owner socket'a + cleanup'ит при disconnect. */
         int r = handshake_subscriber(pub, client);
         if (r != CUFRAMES_OK) {
             close(client);
         }
-        /* TODO v0.2: track client fds для broadcast SHUTDOWN. Сейчас clients
-         * сами detect socket EOF при publisher_destroy через shutdown(). */
     }
     return NULL;
 }
@@ -764,7 +794,23 @@ static int handshake_subscriber(struct cuframes_publisher *pub, int client_fd) {
 
     CUFRAMES_LOG_INFO("subscriber '%s' connected (bit=%u)", name, bit);
 
-    /* TODO v0.2: spawn per-client thread для liveness/PING/UNSUBSCRIBE.
-     * Сейчас socket остаётся открытым на heap'е до publisher_destroy. */
+    /* Spawn detached monitor thread — owns client_fd, frees bit on socket
+     * close (subscriber container exit / crash). Без этого bitmap утекал
+     * каждый pipeline recreate. */
+    struct sub_monitor_args *m = malloc(sizeof(*m));
+    if (!m) {
+        /* OOM — fallback: leak fd, bit будет released только publisher_destroy */
+        return CUFRAMES_OK;
+    }
+    m->pub = pub;
+    m->fd = client_fd;
+    m->bit = bit;
+    pthread_t monitor_tid;
+    if (pthread_create(&monitor_tid, NULL, subscriber_monitor_thread, m) != 0) {
+        CUFRAMES_LOG_WARN("monitor pthread_create fail — bit %u may leak", bit);
+        free(m);
+    } else {
+        pthread_detach(monitor_tid);
+    }
     return CUFRAMES_OK;
 }