refactor: split monolithic modules and extract shared components

- Remove dead socket.c/socket.h (accumulated duplicate symbols)
- Extract compression.h/c (data_compress/decompress from data.c)
- Extract io.h/c (low-level I/O from protocol.c)
- Extract metadata.h/c (unified metadata wire format from file.c, chunk.c, utils.c)
- Split client.c into client_cli.c (CLI parsing) + client_send.c (send logic)
- Move receiver pipeline threads from server.c to multiprocessing.c
- Move to_disk/file_restore_metadata from utils.c to file.c/metadata.c
- Fix const qualifiers on to_disk and str_dup signatures
- Suppress chown unused-result warning

src/shared/multiprocessing.h

@@ -4,6 +4,7 @@
 #include <threads.h>
 
 #include "config.h"
+#include "file.h"
 #include "queue.h"
 
 typedef struct {
@@ -38,4 +39,7 @@ PipelineContextReceiver *pipeline_context_receiver_create(Config *config,
                                                           Queue *queue_receiver,
                                                           int file_descriptor);
 void pipeline_context_receiver_destroy(PipelineContextReceiver *context);
+File *file_receive(Config *config, int file_descriptor);
+int receive_thread(void *pipeline_context);
+int write_thread(void *pipeline_context);
 #endif