ggml-spacemit: type-gate direct GEMV to q4_0, fuse GDN decode row update

[co-seven]

MUL_MAT direct path (IME reads B straight from DRAM, decode only): restrict to q4_0 and drop the q8_0 arm together with its gemm_n <= 2NB_COLS64 crossover. Paired A/B on K3 shows q8_0 has no winning shape: Qwen3.5-35B dense q8_0 decode GEMVs all fall under the old crossover and still measure 21.8-26.7 GB/s direct vs 28.9-30.8 staged through TCM (tg128 6.71 -> 6.91 with direct disabled), while q4_0 gains from direct (Qwen3-0.6B tg128 57.45 -> 59.05, +2.8%; pp unchanged). 35B carries no physical q4_0 weights after repack, so the gate leaves it on the staged path entirely.

GDN decode: add a fused single-pass row update used for n_tokens <= 4. State rows are independent within a token, so the reference's memcpy + decay/dots pass + update pass (3 reads, 3 writes of the state per head) collapse into one load/store pair per row, with k/q/decay held in vector registers across the row loop; decode reads the input state and writes the output state directly with no intermediate copy. Kernel time at n_tokens == 1 drops 424.6 -> 216.0 us; paired A/B tg128 7.58 -> 7.93 (+4.6%), pp128 unchanged. Chunks with more tokens keep the reference two-pass form: their state working copy is L2-resident so the extra passes already run at the bandwidth floor, and the fused form loses there to register spills and the store-at-row-end reduction dependency stalling row loads on the in-order cores (measured crossover ~8 tokens; chunks are 64, or 16 with kda).