fix(raft): fix election and network issues in raft event loop

[slow2342]

Summary

Fix several issues in the Raft event loop that caused election instability and unreliable message delivery. Replace spin-based ticking with accurate time-based ticking, reorder ready processing to match raft-rs best practices, and add observability for debugging raft issues.

Changes

• src/raft/node.rs — Replace MAX_SPIN loop with Instant-based tick scheduling to prevent tick drift. Reorder ready processing: send leader messages before persisting entries (parallel path per raft-rs docs). Replace for loop over ready with while node.has_ready(). Add tracing for steps, proposals, and leader state updates.
• src/main.rs — Increase heartbeat_tick from 1 (100ms) to 2 (200ms) to reduce heartbeat overhead. Set max_committed_size_per_ready to 1MB (0 means no entries returned).
• src/raft/network.rs — Improve send_message error handling to distinguish RPC errors from task panics. Add structured logging for outgoing raft messages.
• src/raft/rpc.rs — Add tracing for received Vote RPC and InstallSnapshot messages.
• src/raft/handle.rs — Add logging for require_leader checks.
• src/raft/raftrs_store.rs — Add debug logging for entries() and term() store operations.

How to verify

cargo build --release
cargo test
cargo clippy -- -D warnings