feat(client): persist virtual MAC identity

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

crates/lanparty-client-core/Cargo.toml

@@ -6,10 +6,13 @@ edition.workspace = true
 [dependencies]
 anyhow.workspace = true
 bytes.workspace = true
+getrandom.workspace = true
 lanparty-ctrl = { path = "../lanparty-ctrl" }
 lanparty-proto = { path = "../lanparty-proto" }
 quinn.workspace = true
 rustls.workspace = true
+serde.workspace = true
+serde_json.workspace = true
 
 [dev-dependencies]
 rcgen.workspace = true