metal: implement DSv4 Lightning Indexer (#25893)

metal: implement DSv4 Lightning Indexer (#25893)

* metal: implement F16 Lightning Indexer

- Implement GGML_OP_LIGHTNING_INDEXER for 128-dimensional, 64-head inputs
  with F32 queries and weights plus F16 keys and masks.
- Add tiled and tail kernels and test KV lengths around 8- and 64-element
  boundaries.

llama-bench (--mmap 1, -fa 1, -p 512, -n 128; d=0/10k/20k/30k):

Before:
- pp512: 153.73 ± 0.87 t/s
- tg128: 8.91 ± 0.04 t/s
- pp512 @ d10000: 73.90 ± 0.39 t/s
- tg128 @ d10000: 8.66 ± 0.03 t/s
- pp512 @ d20000: 45.83 ± 0.18 t/s
- tg128 @ d20000: 8.26 ± 0.03 t/s
- pp512 @ d30000: 33.40 ± 0.21 t/s
- tg128 @ d30000: 7.94 ± 0.01 t/s

After:
- pp512: 155.19 ± 0.91 t/s
- tg128: 8.95 ± 0.04 t/s
- pp512 @ d10000: 86.95 ± 0.69 t/s
- tg128 @ d10000: 9.00 ± 0.05 t/s
- pp512 @ d20000: 62.01 ± 0.45 t/s
- tg128 @ d20000: 8.68 ± 0.04 t/s
- pp512 @ d30000: 49.18 ± 0.33 t/s
- tg128 @ d30000: 8.60 ± 0.02 t/s

Assisted-by: Codex
* metal: stage Lightning Indexer K tiles

- Stage and dequantize K in F16 threadgroup memory before simdgroup matrix loads.
- Zero-fill partial tiles and guard stores so all KV segments use the same numerical path.
- Support F32, F16, BF16, Q4_0, Q4_1, Q5_0, Q5_1, and Q8_0 K caches.

llama-bench (--mmap 1, -fa on, -p 512, -n 128; d=0/10k/20k):

- pp512: 160.38 +/- 1.01 t/s
- tg128: 9.08 +/- 0.03 t/s
- pp512 @ d10000: 88.37 +/- 0.46 t/s
- tg128 @ d10000: 9.07 +/- 0.04 t/s
- pp512 @ d20000: 62.53 +/- 0.46 t/s
- tg128 @ d20000: 8.84 +/- 0.03 t/s

Assisted-by: Codex
* dedup Lightning Indexer constants, fix flaky test

* cont : fix whitespace

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Co-authored-by: forforever73 <redacted>
Co-authored-by: Georgi Gerganov <redacted>