fix: Break combinational loop in desc64 speculation FIFO

[DanielKellerM]

Summary

Fixes #71.

• Disable fall-through (FALL_THROUGH=0) on i_speculation_fifo in idma_desc64_ar_gen_prefetch to break a combinational loop through the speculation check path
• The loop path is: staging_addr_valid_speculation → speculation_valid (fall-through) → speculation_check_addr mux → speculation_correct → flush → staging_addr_valid_speculation
• Under specific conditions (descriptor next-pointer skipping forward within a sequential layout), this reduces to flush = !flush — a combinational oscillator
• The fix adds 1 cycle of latency to speculation checking; actual prefetch performance is unaffected since ARs are already in-flight

Test plan

• tb_idma_desc64_top (558 descriptors, 0 errors)
• tb_idma_desc64_bench (1000 descriptors, 0 errors)
• CI (Questa + VCS)