5. Faults travel with the instruction and are taken at commit

Context. An instruction that faulted at execute marked its ROB entry and squashed everything younger from execute; commit then flushed again when it took the trap. On O3 a trap-carrying result from a non-memory unit completed its ROB entry without faulting it, so debug-trigger breakpoints were lost.

Decision. A fault raised at execute (illegal instruction, ECALL, a debug trigger, or a trap carried from fetch or decode) travels on the execute result. Writeback records it on the ROB entry, and commit takes it and flushes everything younger once, as a real core and gem5 do. The same holds for both backends.

Consequences. Instructions younger than a fault may execute before it is taken; they never retire. A fault costs one flush instead of two. Regression tests: tests/integration/execute_trigger.rs, fault_precedence.rs, trap_latency.rs.