fix: five small guards (NextN post-load, MLA draft warmup hook, zero-expert Marlin, deep_gemm)

[architehc]

Five independent small fixes found while serving GLM-5.2 with kt-kernel CPU experts:

1. NextN draft model: `run_post_load` passthrough (post-load hooks never ran for the draft weights).
2. Deepseek weight loader: fused `weight_shape` metadata merge (loading MTP checkpoints with fused shards).
3. `FlashInferMLAMultiStepDraftBackend`: `on_after_cuda_graph_warmup_pass` hook so draft backends can finalize after warmup.
4. Marlin MoE repack: skip when a layer has zero GPU experts (crash when all experts are CPU-offloaded).
5. `configure_deep_gemm_num_sms`: no-op when deep_gemm was never imported (NameError on the dual-stream decode path with JIT deepgemm disabled).

🤖 Generated with Claude Code

[gemini-code-assist]

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