vulkan: fix SSM_CONV PP scaling with large ubatch sizes (#20379)

vulkan: fix SSM_CONV PP scaling with large ubatch sizes (#20379)

* vulkan: optimize SSM_CONV workgroup dispatch for large ubatch

Tile tokens into 2D workgroups (32x16) to reduce workgroup launch
overhead at large ubatch sizes. Add vec4 fast path for nc=4 (common
d_conv size). Fixes PP performance degradation with ubatch > 512.

Ref: ggml-org/llama.cpp#18725

Co-Authored-By: Claude Opus 4.6 <redacted>
* vulkan: remove unused shared memory declaration in SSM_CONV

Co-Authored-By: Claude Opus 4.6 <redacted>
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Co-authored-by: Progeny Alpha <redacted>
Co-authored-by: Claude Opus 4.6 <redacted>