Add WAL persistence to VSR log

Port the Write-Ahead Log implementation from raft/ into the VSR
server. Log entries are now written to disk with FNV-1a checksums
and fsynced before updating in-memory state. On startup, the WAL
file is loaded and validated, replacing the boot marker for crash
detection. View changes and recovery use atomic rename (tmp.log ->
vsr.log) to replace the WAL safely.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/log.h

@@ -1,3 +1,4 @@
+#if 0
 #ifndef LOG_INCLUDED
 #define LOG_INCLUDED
 
@@ -27,4 +28,5 @@ void log_free(Log *log);
 void log_move(Log *dst, Log *src);
 int  log_append(Log *log, LogEntry entry);
 
-#endif // LOG_INCLUDED
+#endif // LOG_INCLUDED
+#endif