test(decode): verify backpressure and complex drain

[FU-max-boop]

Summary

• Sample DecodeStage ready/valid handshakes before the clock edge and advance each input lane only when it is accepted.
• Drain complex multi-uop instructions through lastUop, with timeout, duplicate, causality, and overflow checks plus per-cycle statistics.
• Add deterministic all-lane backpressure, complex-drain, and illegal-instruction tests, and document the uniform-ready contract and disassembly-oracle boundary.

Why

The previous runner advanced from post-edge ready, consumed valid-only outputs, stopped after all inputs were accepted, and then stepped ten unchecked cycles. That could double-count stalled outputs, miss tail uops, and report false passes. The broad Decode Bundle binding also caused lane-write aliasing with current Picker/Toffee.

Validation

• Current dependency heads: PyToffee f57dd6ff, toffee-test abec0edd: 6 passed with functional coverage enabled.
• Compatible dependency commits: PyToffee f215fa1, toffee-test 4ca2605: 6 passed.
• python3 -m py_compile ut_backend/ctrl_block/decode/env/decode_wrapper.py ut_backend/ctrl_block/decode/test_rv_decode.py
• git diff --check

Test command:

python3 run.py --config "" --no-waveform --no-code-cov -- -q ut_backend/ctrl_block/decode

Validated with the native ARM64 Picker 0.9.0 release against XiangShan RTL commit 66e9b546.