fix(riscv): reject reserved CBO funct7 encodings

[carlosqwqqwq]

Closes #257

Summary

The Zicbom / Zicboz instructions are encoded with a full 7-bit funct7 field of 0b0000000. The affected decoder only checked the top five bits of funct7 ((w >> 27) & 0x1f == 0), so any encoding whose funct7[1:0] is nonzero — a reserved encoding that must raise an illegal-instruction exception — was still decoded as cbo.inval / cbo.clean / cbo.flush / cbo.zero. QEMU raises SIGILL for the same encoding (0x0605200f) because its decoder matches the full 7-bit funct7 pattern.

The fix checks the full 7-bit funct7 in the CBO decode path.

Validation

• cargo test --lib: the targeted CBO decode tests pass, 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 I’m closing this PR as superseded by #296 so the fixes can be reviewed and merged as one coherent change.