Tiered CUDA acceleration for routed experts (opt-in, CPU default untouched) + REPLAY fixture harness (#16)

* feat: add experimental CUDA backend for resident tensors

* feat: promote pinned experts to a bounded VRAM tier

* feat: preload the GPU expert tier at startup

* fix: harden CUDA backend failure handling

* feat: add deterministic multi-GPU tensor placement

* test: add deterministic CUDA benchmark fixture

* perf: make routed experts the default CUDA path

c/backend_cuda.h

@@ -0,0 +1,49 @@
+#ifndef COLIBRI_BACKEND_CUDA_H
+#define COLIBRI_BACKEND_CUDA_H
+
+#include <stddef.h>
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#define COLI_CUDA_MAX_DEVICES 16
+
+/* Opaque, persistent device copy of one resident quantized tensor. */
+typedef struct ColiCudaTensor ColiCudaTensor;
+
+/* Devices are CUDA ordinals, not positions in the input list. */
+int coli_cuda_init(const int *devices, int count);
+void coli_cuda_shutdown(void);
+int coli_cuda_device_count(void);
+int coli_cuda_device_at(int index);
+int coli_cuda_mem_info(int device, size_t *free_bytes, size_t *total_bytes);
+/* device < 0 returns aggregate statistics for all configured devices. */
+void coli_cuda_stats(int device, size_t *tensor_count, size_t *tensor_bytes);
+
+/* Upload without executing, so capacity failures happen during model startup. */
+int coli_cuda_tensor_upload(ColiCudaTensor **tensor,
+                            const void *weights, const float *scales,
+                            int fmt, int I, int O, int device);
+
+/*
+ * y[S,O] = x[S,I] @ W[O,I]^T.
+ * fmt matches QT in glm.c: 0=f32, 1=int8, 2=int4, 3=int2.
+ * The first successful call uploads W and its row scales; later calls reuse it.
+ * Returns 1 on success and 0 when CUDA is not initialized or the format is invalid.
+ */
+int coli_cuda_matmul(ColiCudaTensor **tensor,
+                     float *y, const float *x,
+                     const void *weights, const float *scales,
+                     int fmt, int S, int I, int O, int device);
+
+void coli_cuda_tensor_free(ColiCudaTensor *tensor);
+size_t coli_cuda_tensor_bytes(const ColiCudaTensor *tensor);
+int coli_cuda_tensor_device(const ColiCudaTensor *tensor);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif