Remote clients need a stable locally administered MAC address so the relay,
gateway, DHCP lease, and LAN peers keep seeing the same tunnel identity across
runs. Requiring users to pass `--virtual-mac` made that responsibility manual.
Add a platform-neutral client identity store that loads a JSON identity file or
generates a new valid virtual MAC with OS randomness and persists it. The file
stores the MAC in the same string form shown by the CLI. The Windows client now
uses `lanparty-client-identity.json` by default while keeping `--virtual-mac` as
a manual test override.
TAP binding still remains future work; this slice only owns the client identity
that will be assigned to the TAP adapter.
Test Plan:
- cargo fmt --check
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check
Refs: PLAN.md MAC identity
ddidderr
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