refactor(gemm): migrate the dense matmul family into kernels/gemm/

[ekryski]

What

Wave-2 family port (part 1): migrates the dense matmul kernels into
kernels/gemm/ per docs/specs/KERNEL_CONSOLIDATION_PLAN.md. gemm is the
largest family and is deeply quant-entangled, so it's split: this PR moves the
non-quantized ops; the quantized op×format matrix follows.

Changes

• mlx/{gemv,gemv_masked} + mlx/steel/gemm/* → kernels/gemm/ (with a
  steel/ subdir). Already mt_*-named.
• ffai/{gemm,patch_embed,patch_embed_mma,gemv_axpy_inplace} → kernels/gemm/
  with mt_ prefixes: mt_gemm/mt_gemm_batched, mt_patch_embed/mt_patch_embed_mma,
  mt_gemv_axpy_inplace.
• Fixed the cross-family ffai/ssm.rs test ref and the steel_msl_snapshots
  import; refreshed ffai_gemm doc-comment references; updated the KERNEL_AUDIT
  paths/names and marked gemm dense done in the plan.

Generated MSL per kernel is unchanged — only module paths + inventory names.

Deferred (follow-up)

The quantized matmuls — quantized_* / fp_quantized_* / block_scaled_qmm*,
gemm_q4/q8, gemv_q8, and the batched *_qgemv/*_qmm (+ block-scaled) forms —
are the quantized form of these same ops. They land in kernels/gemm/ in a
follow-up PR; the format-axis fold (plan §7) stays deferred, same as norm/.

Verification

• cargo build + cargo clippy --workspace --all-targets clean.
• Kernel tests pass via tile test: gemm (12), gemv (591 incl. quantized still
  in ffai), gemv_axpy (12), patch_embed (195), steel fused/gather/masked/segmented/splitk (60).
• steel_msl_snapshots (7) and the ssm correctness test pass.

Note on stacking

Stacked on ops (#18) → sampling (#17) → norm (#16) → conv+rope (#15) → dev; the
diff includes those until they merge.

[TheTom]

@ekryski clean mass-move, signatures + MSL identical. Heads-up: patch_embed/patch_embed_mma were previously un-prefixed, so the registered name changes (mt_patch_embed*), easy to miss in a ffai_→mt_ sweep. The pub use … as conv back-compat shim in mlx/steel/mod.rs is a good follow-up delete target once mlx/ fully dissolves. Approve.