feat(fast_io): add splice/vmsplice zero-copy path for network-to-disk transfers

[oferchen]

Summary

• Add vmsplice(2) support for zero-copy userspace-buffer-to-file transfer via try_vmsplice_to_file(), complementing the existing splice(2) socket-to-file path
• Make SplicePipe public with configurable pipe buffer size (default 1MB via fcntl(F_SETPIPE_SZ)) and add splice_to_file()/vmsplice_to_file() instance methods for pipe reuse across transfers
• Add EINTR handling to all splice/vmsplice loops (previously missing - signals could cause spurious failures)
• Refactor internal splice logic into splice_fd_to_file_via_pipe() and drain_pipe_to_fd() helpers shared by both the free functions and SplicePipe methods
• Provide non-Linux stub implementations returning ErrorKind::Unsupported for cross-platform compilation
• Replace benchmark placeholder with real splice_transfer and vmsplice_transfer benches alongside the read_write_baseline

Closes #1361.

Test plan

• CI passes on Linux (splice/vmsplice tests exercise the real syscalls)
• CI passes on macOS (non-Linux stub tests verify Unsupported errors for try_splice_to_file, try_vmsplice_to_file, SplicePipe::new())
• CI passes on Windows (non-unix stub tests verify Unsupported for recv_fd_to_file)
• Verify cargo bench -p fast_io --bench splice_pipe runs on Linux and reports throughput for all three strategies