refactor(spec): split and simplify Raft specification

The original Raft TLA+ model is extracted into a new file `Raft_Vanlightly.tla` to preserve the unmodified version, while `Raft.tla` is streamlined for model‑checking by removing notes, history variables, and consolidating actions. Minor comment clarification is added to `raft.specl` regarding symbolic mode limitations.

Author: danwt