metal : per-op source split + parallel compile (#24021)
* preliminary extract common header
* op source split
* split metallib into 8 libs && load in parallel
* derive kernel->library routing from functionNames
* x-macro lib list + underscore filenames, dedup QK_NL, MRC fixes
* op source split 8 to 20
* improve robustness of source fallback
* clean up
* change bool -> atomic_bool
* only prepend headers that source actually includes
* no semaphore, use GCD global queue
* dedup library compile path, fix NSError lifetime, rename gla
* relocate upstream concat/rope_back/repeat kernel changes into split files
* move ggml-common.h from common.h into dequantize.h to shrink binary size
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Co-authored-by: lvyichen <lvyichen@stepfun.com>
