Complete WAL implementation with file rotation

This commit completes the Write-Ahead Log (WAL) implementation for the
metadata server, including all missing functionality and error fixes:

File System Changes (src/file_system.c):
- Implemented file_set_offset() using lseek (Linux) and SetFilePointer (Windows)
- Implemented file_get_offset() to get current file position

WAL Header Changes (src/wal.h):
- Added file_tree pointer to WAL structure for snapshot serialization
- Added file_path to WAL structure for rotation operations

WAL Implementation (src/wal.c):
- Fixed handle assignment bug in wal_open (was using wal->handle before assignment)
- Fixed missing return statement in append_begin()
- Implemented serialize_callback for writing file tree snapshots to WAL
- Implemented deserialize_callback for reading file tree snapshots from WAL
- Implemented next_entry() to read WAL entries from file during recovery
- Implemented wal_append_write() to write file modifications to WAL
- Implemented swap_file() for complete WAL file rotation:
  * Creates temporary file with new snapshot
  * Writes WAL header and serialized file tree
  * Atomically replaces old WAL file
  * Resets entry counter for new rotation cycle
- Added WAL file initialization for newly created files
- Added helper functions: read_exact, read_u8, read_u16, read_u32, read_u64, write_u32
- Fixed typo in comment (Not -> Now)

The WAL now supports:
- Full crash recovery by replaying logged operations
- Automatic file rotation when entry limit is reached
- Atomic file replacement to ensure durability
- Proper file locking throughout rotation process

src/file_system.c

@@ -59,12 +59,40 @@ void file_close(Handle fd)
 
 int file_set_offset(Handle fd, int off)
 {
-    assert(0); // TODO
+#ifdef __linux__
+    off_t ret = lseek((int) fd.data, off, SEEK_SET);
+    if (ret < 0)
+        return -1;
+    return 0;
+#endif
+
+#ifdef _WIN32
+    LARGE_INTEGER distance;
+    distance.QuadPart = off;
+    if (!SetFilePointer((HANDLE) fd.data, distance.LowPart, &distance.HighPart, FILE_BEGIN))
+        if (GetLastError() != NO_ERROR)
+            return -1;
+    return 0;
+#endif
 }
 
 int file_get_offset(Handle fd, int *off)
 {
-    assert(0); // TODO
+#ifdef __linux__
+    off_t ret = lseek((int) fd.data, 0, SEEK_CUR);
+    if (ret < 0)
+        return -1;
+    *off = (int) ret;
+    return 0;
+#endif
+
+#ifdef _WIN32
+    DWORD pos = SetFilePointer((HANDLE) fd.data, 0, NULL, FILE_CURRENT);
+    if (pos == INVALID_SET_FILE_POINTER && GetLastError() != NO_ERROR)
+        return -1;
+    *off = (int) pos;
+    return 0;
+#endif
 }
 
 int file_lock(Handle fd)