feat(ha-heartbeat-committed-propagation): add xce_ha_heartbeat_req sender (task 1.2)

Send the HA heartbeat as OP_nop + the existing HeartbeatMsg body carrying
{term, committed_lsn, committed_config_tag} (senderPriority unused). Generic
xce_nop_req untouched; C-side body support pre-existed (no typemap edit). Req 1.1
producer half. Round-trip test proves the 3 fields survive the wire; lib suite
0 failures under ASAN.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/lib/request.c

@@ -1138,6 +1138,37 @@ int xce_nop_req(endpoint_t* ep)
     return xce_request(ep, &request);
 }
 
+/* xce_ha_heartbeat_req: send the HA heartbeat as an OP_nop request carrying a
+ * HeartbeatMsg body with the primary's committed position. Clone of the nop
+ * send path plus a heartbeatmsg body. Non-blocking, no reply awaited (the
+ * standby's StateReport rides the normal nop reply). senderPriority is unused
+ * (0). Generic xce_nop_req stays body-less. */
+int xce_ha_heartbeat_req(endpoint_t* ep, uint64_t term, uint64_t committed_lsn,
+                         uint64_t committed_config_tag)
+{
+    heartbeat_msg_t hb = {
+        .sender_priority      = 0,
+        .term                 = term,
+        .committed_lsn        = committed_lsn,
+        .committed_config_tag = committed_config_tag,
+    };
+    /* clang-format off */
+    xce_request_t request = {
+        .opcode     = OPCode_nop,
+        .body_which = Request_body_heartbeatmsg,
+        .body       = {
+            .heartbeatmsg = &hb
+        }
+    };
+    /* clang-format on */
+
+    if (ep == NULL) {
+        return -EINVAL;
+    }
+
+    return xce_request(ep, &request);
+}
+
 int xce_hello_req(endpoint_t* ep, char* msg)
 {
     /* clang-format off */