fix(riscv): reject obsolete sfence.vm encoding

[carlosqwqqwq]

Closes #260

Summary

The obsolete sfence.vm encoding 0x10400073 was still decoded as Op::SfenceVm by the system-opcode table in src/isa/riscv/decode.rs. RISC-V marked this encoding obsolete (riscv-isa-manual 20240411, "SFENCE.VM Instruction"); the current privileged ISA redefines it as sctrclr of the Ssctr/Smctr extension, so a plain sfence.vm must raise an illegal-instruction exception. QEMU traps the encoding with SIGILL.

The fix removes the 0x08 if rs2(w) == 0x04 => base(Op::SfenceVm, w) branch so the encoding falls through to Insn::illegal, deletes the now-unused Op::SfenceVm variant and its disassembler case, and updates the existing decode test that asserted Op::SfenceVm.

Validation

• cargo test --lib: the updated decode test (decode_privileged_fence_and_hypervisor_tables) passes, and the full library test suite passes with no regressions.

No new upstream test files are added; this is a source-only change.

[19h]

Thanks for the report and patch. I independently reproduced and validated this issue, then reimplemented the accepted behavior with shared direct/SMIR validation and expanded tests in #296. That rollup includes your co-author credit and asks for your review. I’m closing this PR as superseded by #296 so the fixes can be reviewed and merged as one coherent change.