fix(riscv): canonicalize reserved WARL encodings in four CSRs

[carlosqwqqwq]

Summary

mstatus.MPP, mip, mideleg and medeleg are WARL CSRs: writes to reserved fields must be canonicalized to the legal value set. RAX's csr_write stored the raw value, so reserved encodings (MPP=0b10, mip[0], mideleg[0], medeleg[10]) became readable.

This change masks the WARL fields in csr_write for these four CSRs: MPP to the implemented privilege set, mip/mideleg to the writable interrupt bits, and medeleg to the delegatable exception causes.

Validation

• New regression tests cover each reserved field: MPP=0b10 canonicalizes to U, mip[0]/mideleg[0]/medeleg[10] clear to 0, and legal values are preserved.
• cargo test --lib targeted tests pass; full suite shows no regressions.

Closes #307

[carlosqwqqwq]

The test-core (linux-x64) failure is unrelated to this patch: it fails in the x86-64 evex_fma3_* / legacy_sha_replay native-replay tests (SIGILL or SHA assertion), while this PR only touches RISC-V CSR code. The same tests pass locally (full cargo test --lib on master + this patch) and on the linux-arm64/macos lanes.

Root cause: this PR's CI merge base is master, which does not yet contain the scalar EVEX FMA3 LLIG canonicalization from #296 (src/smir/ir/x86_native_replay/classifiers/evex_scalar_fma_llig.rs). Until #296 lands, the native-replay tests are unstable on hosted linux-x64 runners. Once #296 is merged, these lanes should be stable for every PR based on master.