fix(riscv): reject vadc/vsbc with vd == v0

[carlosqwqqwq]

fix(riscv): reject vadc/vsbc with vd == v0

Closes #193

Summary

The RISC-V V extension reserves vadc/vsbc when the destination aliases v0, because the carry/borrow-in is read from v0 in every implemented form. vd == v0 now raises an illegal-instruction trap.

Validation

• CPU regression covers reserved vd == v0 and legal controls for vadc.vv/vadc.vxm/vadc.vim and vsbc.vv/vsbc.vxm (vsbc has no immediate form).
• cargo test --lib: cpu suite 72 passed, 0 failed.

Notes

The vector semantics moved to cpu/vector.rs in #202 so this change does not grow the oversized cpu.rs.

[19h]

Review findings:

1. The shared vd == 0 check looks semantically correct, and the focused unit test passed locally (1 passed; 0 failed). The regression test only exercises the vv encodings, though, while vadc/vsbc are accepted with vd == v0 #193 covers all implemented forms. Please add reserved and legal controls for vadc.vxm, vadc.vim, and vsbc.vxm as well. (vsbc.vi does not exist.)

2. Repository policy note: this PR adds to src/isa/riscv/cpu.rs, which is currently 7,221 lines / 264,801 bytes and exceeds both hard split triggers in AGENTS.md. The touched vector semantic group should be split here or in a prerequisite shared refactor.

[carlosqwqqwq]

Thanks for the review. Updated as suggested:

• Regression extended beyond the vv encodings: reserved vd == v0 and legal controls now cover vadc.vxm, vadc.vim, and vsbc.vxm (vsbc.vi correctly omitted) alongside vadc.vv/vsbc.vv.
• Repository policy: the touched vector semantic group is split into src/isa/riscv/cpu/vector.rs in the prerequisite refactor refactor(riscv): split vector semantics out of cpu.rs #202; this PR no longer grows cpu.rs.

Verified locally: cargo test --lib --no-default-features --features smir-jit cpu suite 72 passed, 0 failed.

[19h]

Thanks @carlosqwqqwq. This fix is being consolidated into rollup PR #203, where it shares centralized direct-interpreter, SMIR/JIT, and QEMU differential coverage with the related RVV corrections. Closing this PR in favor of #203; the relevant rollup commits credit you as co-author.