backend: dedup consecutive identical ggml log lines

Install a ggml log callback that collapses exact consecutive
duplicates and reports the total count when the run ends. The CUDA
graph capture logs one reused line per replay step, flooding stderr
and stalling a reader that blocks on a full pipe.

src/backend.h

@@ -8,6 +8,7 @@
 #include "ggml-backend.h"
 #include "qt-error.h"
 
+#include <cstdio>
 #include <cstdlib>
 #include <cstring>
 #include <string>
@@ -66,7 +67,37 @@ static ggml_backend_t cpu_backend_new(int n_threads) {
 // Subsequent calls reuse the same backend (single VMM pool). Returns a
 // BackendPair with .backend == NULL when initialisation fails; the caller
 // must check this before passing it to any pipeline_*_load.
+// Collapse exact consecutive duplicate ggml log lines and report the total
+// count when the run ends (tames the CUDA graph capture "reused" flood).
+static void qt_ggml_log(enum ggml_log_level level, const char * text, void * user_data) {
+    (void) level;
+    (void) user_data;
+    static char last[256] = { 0 };
+    static int  count     = 0;
+
+    if (count > 0 && strcmp(text, last) == 0) {
+        count++;
+        return;
+    }
+
+    if (count > 1) {
+        fprintf(stderr, "[Dedup] Previous line repeated %d times total\n", count);
+    }
+
+    fputs(text, stderr);
+    strncpy(last, text, sizeof(last) - 1);
+    last[sizeof(last) - 1] = 0;
+    count                  = 1;
+    fflush(stderr);
+}
+
 static BackendPair backend_init(const char * label) {
+    static bool log_installed = false;
+    if (!log_installed) {
+        ggml_log_set(qt_ggml_log, nullptr);
+        log_installed = true;
+    }
+
     if (g_backend_refs > 0) {
         g_backend_refs++;
         qt_log(QT_LOG_INFO, "[Load] %s backend: %s (shared)", label, ggml_backend_name(g_backend_cache.backend));