DMA cocotb gap coverage

Directed tests targeting the long-transfer, byte-strobe, unaligned, IRQ, and bus-error gaps tracked in verify/rtl_gap_work_order.yaml#areas.dma.critical_gaps.dma-real-memory-system.

These tests follow the DUT port convention of rtl/dma/e1_dma.sv. They are not yet wired into verify/cocotb/Makefile; integration is tracked as part of the same work order entry.