Optimize implementation, add multi stream support

[timkhronos]

Overview

Fully rewrote minimax-m3.cpp for speed and buffer size gains:

Unified the 4-way + decode, 1 FA call per layer instead of 4, with the groups mapped onto ne[3]

Custom CPU op now emits block-level mask, expanded on GPU, which causes CPU to GPU transfer to shrinks at prefill

Decode: ~25 nodes/layer vs ~50, no per-group concats/conts

Unified selection semantics, so both regimes rank bs + local bias (position-anchored local force), which means prefill/decode can no longer disagree on selection

can_reuse on the MSA bias input. Graph reuse at decode restored (was rebuilding the full graph every token)

In-place mask adds, shrinking compute buffer ~6.8 to ~4.2 GiB at ub2048/62k

Multi-stream: MSA now runs with -np N when kv_unified=false. Decode stays batched across streams (still 1 FA call), prefill loops per stream. dense fallback only for --kv-unified + multi-seq

Additional information

src/llama-kv-cache.cpp: k_idx cache f32 -> f16

src/models/models.h: two MSA method decls, instead one build_attn_msa_fa(…, Gp, …)

Measured effect: decode 6.2(4WAY)–7.15(MSA_decode) -> 7.7~7.8 t/s, flat from 5k to 60k+. prefill around 10% faster. buffer about 20% smaller, multi-user support.

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