Describe the bug

After unlocking PMP and mret into S-mode, a second mret does not trap. NutShell still performs M-mode mret (updates mstatus and drops to U-mode).

# MPP=S, MPIE=1
mret                 # enter S-mode
# in S-mode:
mret                 # must be illegal; DUT still executes it

Expected behavior

RISC-V Privileged Spec, Trap-Return Instructions: mret is valid only in M-mode. Executing it at a lower privilege must raise an illegal-instruction exception.

• Expected: mcause=2, mtval=0x30200073
• Actual: no trap; privilege becomes U-mode

In CSR.scala, wen = (valid && func =/= CSROpType.jmp) and
csrExceptionVec(illegalInstr) := (isIllegalAddr || isIllegalAccess) && wen.
mret uses func=jmp, so wen=0 and the privilege check is skipped, while when (valid && isMret) still runs.

Environment

NutShell: 041f694