NPU cocotb gap coverage

Scaffolds for the v0 NPU descriptor-queue ABI documented in docs/arch/npu-microarch.md. Tests are decorated with skip=True until the e1_npu_gemmini_wrapper RTL lands; they encode the queue, IRQ, and unsupported-op contract that wrapper must satisfy.

Tracked under verify/rtl_gap_work_order.yaml#areas.npu.critical_gaps.npu-production-accelerator and npu-test-coverage-accounting.