docs(testing): add local MVP preflight

The manual MVP guide now has hardware pass conditions, but it did not give a
single local command set to run before moving to Windows and the LAN. Add a
short preflight section with the format, workspace test, clippy, and Linux
release-build commands.

The section also names what the workspace tests cover locally: DHCP-shaped,
ARP-shaped, ping-shaped, UDP broadcast discovery, and gateway restart session
paths. It explicitly keeps the Windows TAP driver, real LAN DHCP, switch MAC
learning, and real game proof as manual acceptance work.

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

@@ -103,6 +103,23 @@ where.exe link
 
 `rustc -vV` should report a `host` containing `x86_64-pc-windows-msvc`.
 
+## Local Preflight
+
+Run these before moving to the hardware test:
+
+```bash
+cargo fmt --check
+cargo test --workspace
+cargo clippy --workspace --all-targets -- -D warnings
+cargo build --release -p lanparty-relay -p lanparty-gateway
+```
+
+The workspace tests include real client/relay/gateway session coverage for
+DHCP-shaped traffic, ARP-shaped traffic, ICMPv4 ping-shaped traffic, UDP
+broadcast discovery traffic, and gateway restart lifecycle. They do not prove
+the Windows TAP driver, real LAN DHCP, physical switch MAC learning, or a real
+game. Those are the manual pass conditions below.
+
 ## Build
 
 On the relay or Linux build host: