The Windows client frame pump needs one task reading TAP frames while another
writes relay frames back to the adapter. The TAP crate already owns the raw
Windows file handle; this change makes the handle's thread-safety boundary
explicit with `Send` and `Sync` impls and documents the Windows handle
assumption next to the unsafe code.
This does not add the pump yet. It only prepares the adapter type for the
separate read/write loops that will wire TAP I/O to the relay session.
Test Plan:
- cargo fmt --check
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- CC_x86_64_pc_windows_msvc=clang-cl AR_x86_64_pc_windows_msvc=llvm-lib \
CARGO_TARGET_X86_64_PC_WINDOWS_MSVC_LINKER=lld-link cargo clippy \
-p lanparty-client-tap --target x86_64-pc-windows-msvc -- -D warnings
- git diff --check
Refs: PLAN.md
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