graph : remove redundant GDN state transposes (#20443)

* ggml : transpose fused GDN state access for coalesced memory reads (#20436)

The fused Gated Delta Net kernel accessed the [S_v, S_v] state matrix
column-wise on row-major storage, causing strided reads (stride S_v =
128 floats = 512 bytes) that waste GPU cache bandwidth. This produced a
39% regression on Qwen3.5-9B (Metal, M4 Max) compared to the unfused
path.

Transpose the state indexing so threads read contiguously:
- Metal: s_ptr[is*S_v] -> s_ptr[is] (stride 1 vs S_v)
- CUDA:  curr_state[i*S_v+col] -> curr_state[col*S_v+i] (coalesced)
- CPU:   restructured loops for row-wise transposed access

Also add --fused-gdn [on|off|auto] CLI flag (mirrors --flash-attn) so
users can control fused GDN independently of auto-detection.

All GATED_DELTA_NET backend-ops tests pass.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* ggml : use SIMD dot products in CPU GDN kernel, couple AR/chunked fused flags

- Replace scalar inner loops with ggml_vec_dot_f32 for SIMD-optimized
  dot products in the CPU fused GDN kernel (delta and attention output)
- Couple fused_gdn_ar and fused_gdn_ch flags in auto-detection: if one
  path lacks device support, disable both to prevent state layout mismatch
  between transposed (fused) and non-transposed (unfused) formats

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* llama : rever fgdn argument changes

* graph : remove GDN state transposes

* vulkan : adapt

* cuda : remove obsolete smem code

---------

Co-authored-by: Paul Flynn <paul@arkavo.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Oliver Simons <osimons@nvidia.com>

ggml/src/ggml-vulkan/vulkan-shaders/gated_delta_net.comp

@@ -44,7 +44,7 @@ void main() {
 
     FLOAT_TYPE state[S_V];
     [[unroll]] for (uint i = 0; i < S_V; i++) {
-        state[i] = FLOAT_TYPE(data_state[state_base + i * S_V + col]);
+        state[i] = FLOAT_TYPE(data_state[state_base + col * S_V + i]);
     }
 
     uint attn_off = (seq_id * n_tokens * H + head_id) * S_V;
@@ -123,6 +123,6 @@ void main() {
     }
 
     [[unroll]] for (uint i = 0; i < S_V; i++) {
-        data_dst[s_off + state_base + i * S_V + col] = state[i];
+        data_dst[s_off + state_base + col * S_V + i] = state[i];
     }
 }