ggml-rknpu2: graceful abort with W8A8 hint on NPU IOVA exhaustion

rknn_create_mem returns null when the RK3588 NPU's ~4 GiB IOVA aperture is
full -- this backend maps every weight tensor into a single 4 GiB IOMMU window
(rkopnu currently ignores iommu_domain_id, so the IOMMUDomainManager's 2 GiB
domain-spreading is a no-op and everything lands in one 4 GiB window). The old
GGML_ASSERT reported a cryptic "Failed to allocate tensor memory via RKNN API"
with no guidance.

Replace it with a GGML_ABORT that names the aperture and points at the fix:
use a quantized GGUF, or set RKNPU_HYBRID=W8A8_STANDARD for on-the-fly INT8
weights (which fits >4 GiB dense f16 models -- e.g. Qwen2.5-3B-f16 runs at
~130 t/s pp512 that way). Also null-check the grow re-alloc path, which was a
latent copy of the same crash.

No behaviour change for models that fit; only the failure message improves.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EWpfhDgYNA21tETDP9ueBE

ggml/src/ggml-rknpu2/ggml-rknpu2.cpp

@@ -296,6 +296,9 @@ struct ggml_backend_rknpu_buffer_context {
                 it->second.iommu_domain_id = g_domain_manager.assign_domain_memory(size);
                 rknn_matmul_ctx new_ctx = g_domain_manager.get_allocator_context(it->second.iommu_domain_id);
                 it->second.mem = rknn_create_mem(new_ctx, size);
+                if (it->second.mem == nullptr) {
+                    GGML_ABORT("rknpu2: rknn_create_mem failed for %zu bytes (grow re-alloc) -- the NPU's ~4 GiB IOVA aperture is exhausted; this backend maps all weights into a single 4 GiB IOMMU window. Use a quantized GGUF, or set RKNPU_HYBRID=W8A8_STANDARD for on-the-fly INT8 weights.", size);
+                }
                 it->second.size = size;
             }
             return it->second;
@@ -311,7 +314,9 @@ struct ggml_backend_rknpu_buffer_context {
         alloc.size = size;
         alloc.iommu_domain_id = domain_id;
 
-        GGML_ASSERT(alloc.mem != nullptr && "Failed to allocate tensor memory via RKNN API");
+        if (alloc.mem == nullptr) {
+            GGML_ABORT("rknpu2: rknn_create_mem failed for %zu bytes -- the NPU's ~4 GiB IOVA aperture is exhausted; this backend maps all weights into a single 4 GiB IOMMU window. Use a quantized GGUF, or set RKNPU_HYBRID=W8A8_STANDARD for on-the-fly INT8 weights.", size);
+        }
         tensor_allocs[tensor_offset] = alloc;
 
         return alloc;