refactor(moe): migrate the moe family into kernels/moe/

[ekryski]

Stacked on #24. Migrates the moe family — the bulk of wave 3 — from ffai/ + mlx/ into kernels/moe/ (which #24 seeded with gather_q4 + sigmoid_bias). Two commits, as agreed: move whole, then split the monster file.

Commit 1 — migrate (35 files)

Group	Modules
Routing	orchestration (ex moe.rs), router_topk_biased (ex dsv4_router_topk), router_sigmoid_bias, router_sqrtsoftplus
MPP grouped BGEMM	mpp(+_int8/_bm8/_bm64/×int8/×block_scaled) + mpp_shared
GGUF-format expert matmul	bgemm_{q2k,iq2xxs,q4}_*, gemv_{rows,ws,rows_view}_*, gather_{down_q2k,gemv_iq2xxs}
Down combine	down_swiglu_accum, down_weighted_sum_f16
Expert-indexed / block-scaled	dequant_gemv_expert_indexed(_block_scaled), block_scaled (ex mlx/block_scaled_moe)

• Filenames drop the redundant moe_ prefix (folder provides it; matches refactor(gemm): split the gemv_q8 grab-bag by family #24); kernel names keep mt_moe_*.
• Model-name purge: mt_dsv4_router_topk → mt_moe_router_topk_biased — it's a distinct kernel from the generic mt_moe_router_topk (selects top-k by the biased score, weights by the unbiased). Bare dequant_gemv_int4_expert_indexed → mt_ prefixed.
• Fixed the mpp_shared intra-imports and ~27 consumer test files (grouped + single-line-mixed use blocks included).
• Format-axis fold (§7) deferred — the 13-kernel *_block_scaled matrices move as-is.

Commit 2 — split orchestration.rs

The former moe.rs (~4.3k lines, 13 kernels) moved whole in commit 1, then split per the <1000-line guideline:

• router_topk.rs — mt_moe_router_topk
• permute.rs — mt_moe_permute / mt_moe_unpermute
• gather_qmm.rs — the 10 grouped-gather quantized-matmul cells + their CSR/MMA test/bench helpers

Each file carries its own kernel_tests + kernel_benches; consumer imports remapped orchestration:: → the new modules.

Verification

• cargo build --workspace --bins ✓
• cargo clippy --workspace --all-targets --all-features -- -D warnings ✓ (clean)
• tile build — 1272 kernels codegen to MSL (count unchanged across both commits); every renamed/relocated/split kernel tile inspect-able
• 27 moe integration suites pass (59 tests, 0 failed); in-file kernel tests green (gather_qmm 315, router_topk 6, permute 6, expert_indexed 93)

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[TheTom]

@ekryski clean move + legit int4/int8 mpp fold (6→3 files via variants, expanded names byte-for-byte). mt_dsv4_router_topk→mt_moe_router_topk_biased is fully propagated (stale test filename only). Approve.