ggml-metal: add chunked SSD MMA for Mamba-2 prefill optimization (#26647)

* metal: WIP chunked SSD SSM_SCAN kernels for multi-token prefill

* metal: drop scalar SSD path; MMA + sequential tail

* drop WIP ssm scan test noise

* remove state_from_dst and rename CS and NSG constants

* remove unrelated  added whitespace padding

* added clarity to mma_tokens calculation

* added clarity to use_mma bool checks

* added comments to metal ssd op constants for clarity

* reserve K tokens for sequential kernel rollback snapshots

* reset concurrency between mma and seq tail

* remove print args no longer used

* fixed comment to no longer point to specific line

* add FC_SSM_SCAN so seq path skips token offlset unless it's mma tail

* added changes to new ssm.metal for rebase after ggml-metal.metal refactor

* specialize ssm_scan tail with a template instead of a function constant

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Co-authored-by: dpantaleoni <dominikpantaleoni@gmail.com>
Co-authored-by: forforever73 <690105611@qq.com>