fix(riscv): validate vcompress destination alignment

[carlosqwqqwq]

Summary

Reject vcompress.vm encodings whose destination and source register groups are not aligned to their LMUL size. The illegal-condition path now checks both vd % emul != 0 and vs2 % emul != 0, alongside the existing vm, vstart, and source-overlap checks.

Validation

• vcompress_rejects_misaligned_vd: e32,m2 cases with misaligned vd=v1 and misaligned vs2=v1 trap; the aligned control vd=v4, vs2=v8 executes.

Closes #319