docs(testing): include diff check in preflight

The local MVP preflight should include the whitespace/diff hygiene check that is
part of the handoff workflow. Add `git diff --check` to the command block so the
manual test guide matches the verification run before committing or moving to
hardware.

Test Plan:
- cargo fmt --check
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- cargo build --release -p lanparty-relay -p lanparty-gateway
- git diff --check

Refs: MVP manual test handoff

TESTING.md

@@ -112,6 +112,7 @@ cargo fmt --check
 cargo test --workspace
 cargo clippy --workspace --all-targets -- -D warnings
 cargo build --release -p lanparty-relay -p lanparty-gateway
+git diff --check
 ```
 
 The workspace tests include real client/relay/gateway session coverage for