docs(readme): advertise MVP preflight checks

The README entry point still showed only `cargo check --workspace`, while the
manual MVP guide now has a stronger local preflight. Align the README build
section with that handoff so someone starting from the repository front page
runs the same local tests, clippy, Linux release build, and diff check before
moving to Windows and LAN hardware.

The README also now calls out that these checks cover only local Rust and
session paths; the Windows TAP build and final hardware proof remain in
TESTING.md.

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

README.md

@@ -119,13 +119,19 @@ Public relay binary and relay-owned room state:
 - per-peer last-seen timestamps in relay room snapshots
 - peer leave cleanup for room membership and MAC indexes
 
-## Build
+## Build And Local Checks
 
 ```bash
-cargo check --workspace
+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
 ```
 
-For the manual MVP end-to-end proof, see [TESTING.md](TESTING.md).
+These checks cover the local Rust code and the real client/relay/gateway
+session paths that can run without Windows TAP or LAN hardware. For the Windows
+client build and the manual MVP end-to-end proof, see [TESTING.md](TESTING.md).
 
 ## Relay