feat(raft): implement ReadIndex-based linearizable reads

[slow2342]

Summary

Replace the simplified leader-check-only linearizable read with the full Raft ReadIndex protocol. The leader now sends a heartbeat to confirm its authority before allowing reads, preventing stale reads during network partitions where a leader has been isolated but hasn't yet discovered a new leader.

Changes

• src/raft/node.rs — Add ReadIndexRequest type, read-index channel in EventLoopChannels, pending tracking in event loop, and Notify on RaftSharedState for efficient wait signaling.
• src/raft/handle.rs — Add read_index() and wait_for_apply() to RaftHandle trait. Make ensure_linearizable() async to use the real ReadIndex protocol. Add Clone derive to RaftError. Add 12 unit tests.
• src/raft/raftrs_handle.rs — Implement read_index() and wait_for_apply() via channel and shared Notify.
• src/api/kv.rs — Update linearizable_read() to use async ensure_linearizable() and Notify-based waiting instead of 5ms polling. Gate range() on serializable field.
• src/main.rs — Wire read_index_tx from RaftNodeHandle into RaftRsHandle.
• proto/kv.proto — Add bool serializable = 7 to RangeRequest, matching GetRequest.

Testing

• 12 new unit tests covering read_index(), wait_for_apply(), ensure_linearizable() with leader/no-leader/not-leader/channel-closed/redirect scenarios
• All 102 existing tests pass
• cargo clippy -- -D warnings and cargo fmt -- --check clean

Related issues

Refs #84