fix(riscv): reject illegal compressed encodings at the native boundary

[carlosqwqqwq]

fix(riscv): reject illegal compressed encodings at the native boundary

Closes #238

Summary

Under RV64GC, the reserved compressed encoding C.LUI rd=x0 (raw 0x6005) is admitted and retired by the translated/JIT path instead of raising an illegal-instruction trap (cause 2). The hand-written compressed lifter dispatches by quadrant/funct3 and lift_c_lui_addi16sp() produces empty ops for rd=x0; the empty block is accepted by the native path and retired. The same admission gap covers C.ADDIW rd=x0, reserved RV32C shift encodings, and Zcb aliases when M/Zbb/Zba are missing.

The fix rejects Insn::Illegal first at the decoded native boundary so the encoding falls back to the interpreter, which delivers the precise trap and mtval.

Validation

• Targeted test jit_does_not_admit_illegal_compressed_encoding fails on the base checkout and passes with the fix.
• The interpreter fallback delivers the precise trap (cause 2) with mtval for the same encodings.
• cargo test --lib: full library test suite passes with no regressions.

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

[19h]

Thank you for reporting and proposing this fix. I independently reimplemented and validated fail-closed native admission for predecoded illegal compressed instructions across O0 and O2 in the consolidation PR #231; no commits or code from this branch were taken. The rollup carries @carlosqwqqwq as co-author on its commits and includes direct and native regression coverage. I’m closing this PR as superseded by #231—please review the consolidated implementation there.