wasm-riscv-online progressive Integration of riscv-decode

[CAICAIIs]

Problem

Manual decoder is slow and hard to maintain.

Plan

Keep current APIs; replace only the bit-parsing layer with riscv-decode via an adapter.

Pros

• 3-5× faster decoding
• 60 % less decode code
• Zero breaking changes, easy rollback

Cons

• Short-term dual code paths
• Need thin adapter for format conversion
• One extra crate in build

Tasks

1. Add riscv-decode = "0.2.1" to Cargo.toml
2. Add src/decode/riscv_decode_adapter.rs
3. Swap 32-bit decode calls; keep output format
4. Verify tests, WASM build, and perf gain