media_download tests: backpressure, block relay, OOB block_id, assembly

5 new tests in test_media_delivery_download:
- conn_cb starts only first block (backpressure)
- conn_cb with multiple peers
- BLOCK_DONE triggers next block relay from same peer
- assembly after all blocks validated
- peer with more blocks than dl (OOB fix verification)

md_dl_conn_cb made non-static for test visibility
md_dl_send added NULL guard for test instances without routing
media_download.h: added conn_cb extern declaration

src/media_delivery/media_download.c

@@ -57,6 +57,7 @@ static void md_mkdir_parent(const char* filepath) {
 }
 
 static int md_dl_send(struct UTUN_INSTANCE* inst, uint64_t dst, const uint8_t* data, size_t len) {
+    if (!inst || !inst->topo_groups || !inst->connections) { DEBUG_WARN(DEBUG_CATEGORY_GENERAL, "%s: md_dl_send: no routing (inst=%p topo=%p conn=%p)", MDL_ID, (void*)inst, (void*)(inst ? inst->topo_groups : NULL), (void*)(inst ? inst->connections : NULL)); return -1; }
     struct ll_entry* e = queue_entry_new(0);
     if (!e) { DEBUG_ERROR(DEBUG_CATEGORY_GENERAL, "%s: queue_entry_new failed for send to 0x%016llx", MDL_ID, (unsigned long long)dst); return -1; }
     e->dgram = u_malloc(len + 1);
@@ -120,7 +121,7 @@ static void md_dl_send_have_block(struct UTUN_INSTANCE* inst, struct media_downl
 
 /* ── conn_mgr callback ── */
 
-static void md_dl_conn_cb(struct CONN_MGR_HANDLE* h, uint64_t node_id, uint64_t group_id,
+void md_dl_conn_cb(struct CONN_MGR_HANDLE* h, uint64_t node_id, uint64_t group_id,
                            enum conn_mgr_event event, void* arg) {
     struct media_download* dl = (struct media_download*)arg;
     DEBUG_INFO(DEBUG_CATEGORY_DEBUG, "%s: conn_cb event=%d node=0x%016llx dl=%p active=%d num_peers=%d num_blocks=%d",

src/media_delivery/media_download.h

@@ -105,6 +105,12 @@ void media_download_handle_done(struct UTUN_INSTANCE* inst,
 void media_download_handle_overloaded(struct UTUN_INSTANCE* inst,
                                        const uint8_t* data, size_t len);
 
+/* ── conn_mgr callback (non-static for test visibility) ── */
+struct CONN_MGR_HANDLE;
+enum conn_mgr_event;
+void md_dl_conn_cb(struct CONN_MGR_HANDLE* h, uint64_t node_id, uint64_t group_id,
+                    enum conn_mgr_event event, void* arg);
+
 #ifdef __cplusplus
 }
 #endif