feat(raft): safe log compaction with snapshot metadata

- lastSnapshotIndex/Term bound the compacted prefix; lastLogIndex/Term
  and AppendEntries prevLog checks respect the snapshot base
- compactLog drops entries only through min(matchIndex, lastApplied) on
  the leader so lagging peers can still catch up via AppendEntries
- Persist snapshot fields in raft_state.bin; BARADB_RAFT_LOG_MAX_ENTRIES
- Fix commit-index scan to use findLogEntryByIndex (works after compact)

docs/en/distributed.md

@@ -17,9 +17,12 @@ Leader election and log replication over TCP. Enable with:
 | `BARADB_RAFT_PEERS` | Comma-separated `id@host:port` (include self) |
 | `BARADB_RAFT_WRITE_TIMEOUT_MS` | Max wait for majority commit on SQL writes (default 5000) |
 | `BARADB_RAFT_CLIENT_PEERS` | Optional `id@host:clientPort` map for leader write forwarding |
+| `BARADB_RAFT_LOG_MAX_ENTRIES` | Soft cap on in-memory raft log length (default 256); safe prefix compact |
 
 When Raft is enabled, SQL DML (`INSERT`/`UPDATE`/`DELETE`/`MERGE` and transactional `COMMIT`) and schema DDL (`CREATE`/`DROP`/`ALTER` table, index, view, graph, …) are accepted only on the leader of the **`default`** database. DML ships as put/delete log entries; DDL ships as a `ddl` entry with the original SQL and is re-executed on every node at apply. Followers that receive a write/DDL **forward** it to the leader when `BARADB_RAFT_CLIENT_PEERS` maps the leader id to a SQL client address; otherwise they return `not leader; leader is '…'`. Writes against any other database name are rejected (`raft writes only supported on the 'default' database`). `CREATE`/`DROP DATABASE` are not raft-replicated (multi-DB is out of scope for v1). Committed DML also updates secondary B-tree/FTS/HNSW indexes and in-memory graphs.
 
+**Log compaction (v1):** after apply, each node may drop a fully-safe log prefix once `log.len` exceeds `BARADB_RAFT_LOG_MAX_ENTRIES`. The leader never discards past any peer's `matchIndex` (so lagging followers still catch up via AppendEntries). Snapshot metadata (`lastSnapshotIndex`/`Term`) is persisted in `raft_state.bin`; full InstallSnapshot state-machine payloads are not required while this safe-prefix policy holds.
+
 ```nim
 import barabadb/core/raft