from_bytes and to_bytes were using little-endian, causing sendto to
destination 10.0.2.15 to become 15.2.0.10 and break loopback. Changed
both to big-endian (network byte order) per Linux/POSIX standard.

- crates/scarlet/src/net/socket.rs: from_bytes now uses from_be_bytes for
  addr and BE byte order for port; to_bytes writes BE port
  (Open Code assisted, w/ deepseek-v4-pro)
- crates/sakuya/tests/net-loopback/main.c: added loopback E2E test:
  bind(12345) → sendto(10.0.2.15:12345) → recvfrom verifies data
  (Open Code assisted, w/ deepseek-v4-pro)
- crates/sakuya/tests/net-loopback/test.toml: added manifest
  (Open Code assisted, w/ deepseek-v4-pro)