The future client pump should exchange Ethernet frames with TAP through a narrow
API, not raw byte reads and writes. Add TAP frame validation at the adapter
boundary so malformed or jumbo frames are rejected before they enter the relay
path.
Expose `TAP_FRAME_BUFFER_LEN` and `validate_tap_ethernet_frame`, then add
Windows helpers that read a TAP frame and validate it, or validate and fully
write an Ethernet frame. Raw read/write methods remain available for lower-level
adapter work.
Test Plan:
- cargo fmt --check
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- Windows-target cargo clippy for lanparty-client-tap with -D warnings
- git diff --check
Refs: PLAN.md one TAP Ethernet frame per datagram
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