fix(client): reuse existing relay host route

A crashed or forcibly killed Windows client can leave the scoped relay host
route behind. The next run should still be allowed to start when Windows says
that the exact route row already exists, because that route already protects the
relay path from TAP default-route takeover.

Handle ERROR_OBJECT_ALREADY_EXISTS from CreateIpForwardEntry2 as a successful
borrowed pin. Routes created by the current client are still deleted on Drop;
pre-existing routes are left alone so we do not remove administrator-managed or
stale routes that this process did not create. The client startup log now marks
whether the route was created or already existed, and the README and MVP test
guide explain the behavior.

Test Plan:
All cargo commands used these environment variables:
RUSTUP_HOME=/tmp/softlan-vpn-rustup
CARGO_HOME=/tmp/softlan-vpn-cargo

- cargo fmt --check
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- cargo check -p lanparty-client-route --tests
  --target x86_64-pc-windows-gnu
- cargo check -p lanparty-client-route --tests
  --target x86_64-pc-windows-msvc
- cargo clippy -p lanparty-client-route --tests
  --target x86_64-pc-windows-gnu -- -D warnings
- cargo clippy -p lanparty-client-route --tests
  --target x86_64-pc-windows-msvc -- -D warnings
- git diff --check

Known limitation: full lanparty-client-win Windows cross-check is still blocked
on this Linux host by the external ring toolchain setup. The GNU target lacks
x86_64-w64-mingw32-gcc, and the MSVC target lacks lib.exe/MSVC environment.

Refs: PLAN.md route-protection requirement

TESTING.md

@@ -125,6 +125,10 @@ TAP driver reports MAC ... and MTU ...
 client diagnostics: relay reachable yes gateway connected yes route pinned yes ...
 ```
 
+The route pin line ends with `(created)` or `(already existed)`. Either is OK.
+`already existed` usually means a matching relay host route was already present,
+for example after a previous crashed test run.
+
 The first diagnostics line may show `IP unknown`. After DHCP succeeds, a later
 line should show:
 
@@ -231,8 +235,9 @@ Uncommon LAN subnets such as `10.73.42.0/24` are safer than `192.168.0.0/24`.
 
 ## Cleanup
 
-Stop client, gateway, and relay with Ctrl-C. The Windows client restores the TAP
-route policy and marks TAP media disconnected when it exits normally.
+Stop client, gateway, and relay with Ctrl-C. The Windows client removes the
+relay host route only when it created that route itself, restores the TAP route
+policy, and marks TAP media disconnected when it exits normally.
 
 Keep `lanparty-client-identity.json` if you want the same virtual MAC on the
 next run. Delete it only when you intentionally want a new client identity.