linalg/block_quant: W4A8 int8-dot decode GEMV for Q4_0

[czoli1976]

Adds a W4A8 (4-bit weight, int8 activation) decode GEMV on the Q4_0 block-quant format —
the CPU primitive for an int4-compute matmul path, as an alternative to today's
dequantize-to-f32-then-f32-kernel route. Q4_0::w4a8_gemv quantizes the activation row to int8
per block, dots it with the unpacked 4-bit weights, and scales each block by the weight and
activation scales.

Why

The GPU backends already do exactly this — CUDA routes Q4_0 to a Q4_0 × Q8_1 kernel
(cuda/src/kernels/cu/mm_mv_q.cu), Metal to mul_mv_q4_0. On CPU the block-quant path only
dequantizes to f32, so the int4 win is just reduced weight bandwidth, not int8 compute. This is
the missing CPU piece.

Notes / numbers

• Near-lossless: validated against the f32-dequant GEMV over the same Q4_0 weight — the int8
  activation adds ≈0.3% (W4A8 ≈ W4A16); test included.
• No intrinsics, no unsafe: the contiguous i8→i32 inner loop autovectorizes to a NEON
  sdot on its own (integer reductions are associative; strict-IEEE f32 ones aren't).
• Decode-shape (4096², M=1) it measured ~1.8× over mmv_f32 in a local prototype — that includes
  op-wrapper overhead, so a kernel-selection integration would be ≥ that.

Scope / integration

This PR is the kernel primitive only — it is intentionally not yet wired into kernel
selection. The integration question (the W4A8-vs-W4A16 numerical contract, and whether it lands as
a dedicated op like the Metal/CUDA Q4_0×Q8 ops or via strategize in einsum_matmul) is the
open discussion in #2341. Filing the validated primitive + numbers to ground that — happy to wire
it whichever way you prefer.

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