refactor(gemm): migrate quantized matmuls into kernels/gemm/

[ekryski]

Stacked on #22 (ssm). Completes wave 2 of the kernel consolidation (docs/specs/KERNEL_CONSOLIDATION_PLAN.md).

What

Moves the 28 quantized-matmul files from the legacy mlx/ + ffai/ split into kernels/gemm/, alongside the dense GEMM/GEMV set landed in #20:

• Integer-quantized — quantized (+_mpp/_mpp_int8/_nax/_nax_int8/_mma_dynamic_m), dequant_gemv
• FP-quantized — fp_quantized (+_mma/_nax)
• Block-scaled / microscaling — block_scaled_{matmul,mma,qmm,qmm_mpp,qmm_nax}
• Batched projections — batched_{4,qkv}(_block_scaled)_{qgemv,qmm}
• Quantized GEMM — gemm_q8, gemm_q8_mpp, gemm_q4_mpp
• Quantized patch-embed — patch_embed(_mma)_block_scaled

Naming

• Dropped ffai_ prefixes → mt_ (gemm_q8/grouped_gemm_q8(_mpp), gemm_q4_mpp, batched_*, dequant_gemv*). MLX kernels were already mt_-prefixed.
• No model names in kernel names; files keep their format-distinguished names (the format-axis fold, plan §7, is deferred).
• Fixed intra-set path refs (crate::mlx::* → crate::kernels::gemm::*) and all consumer-side test imports.

Scope / parked

• gemv_q8 (1069-line grab-bag spanning gemm/kv_cache/moe/norm/ops) stays parked until moe lands.
• Excluded (migrate with their home family): block_scaled_dequant → quant; block_scaled_moe/moe_* and dequant_gemv_expert_indexed* → moe.

Verification

• cargo build --workspace --bins ✓
• cargo clippy --workspace --all-targets --all-features -- -D warnings ✓ (clean)
• tile test: qmm (966), batched (390), dequant_gemv (114), patch_embed (195), fp_qmm (6), qmv (21) — all pass
• Touched integration tests: gemm_q8(_mpp)/grouped_gemm_q8/qmm_mma_dynamic_m/qmm_mpp(_int8)/dequant_gemv_indirect_wrappers/steel_msl_snapshots — all pass (33 tests)

Docs updated: plan §2/§6 mark gemm/ done; KERNEL_AUDIT paths/names updated.

[TheTom]

@ekryski clean, 28-in/28-out reconciled. One non-#[kernel] public rename to flag in the body: dequant_gemv_wants_indirect → mt_dequant_gemv_wants_indirect (no alias), any runtime caller resolving indirect-dispatch by that symbol breaks. Approve.