docs: expand MVP test guide

Refresh the manual MVP test guide around the current relay, gateway, and
Windows client behavior. The guide now calls out the values to record during a
run, the exact process order, expected gateway lifecycle signals, and a short
gateway restart sanity check.

Also update the README component notes so the control plane and Windows client
docs mention the gateway peer id carried in welcomes.

Test Plan:
- git diff --check
- git diff --cached --check

Refs: MVP manual validation

README.md

@@ -33,7 +33,7 @@ Reliable control-plane schema shared by the QUIC stream handlers:
 
 - endpoint hello messages with role, room, MAC, and datagram budget
 - server welcome mode, reject, peer lifecycle, stats, and disconnect messages
-- initial room gateway-presence status in server welcomes
+- initial room gateway-presence status and gateway peer id in server welcomes
 - room-code, role/MAC, peer-id, and effective-MTU validation
 - length-prefixed JSON control frames for reliable QUIC streams
 
@@ -290,5 +290,5 @@ with direction, peer id, MACs, ethertype/length, frame length, action, and drop
 reason. TAP device read/write errors still stop the bridge.
 Relay lifecycle events are logged as they arrive, including gateway joins and
 peer leaves. The client remembers peer identities from join and catch-up events
-so later leave logs can identify a disconnected LAN gateway or client MAC when
-that peer was known.
+and from the initial welcome, so later leave logs can identify a disconnected
+LAN gateway or client MAC when that peer was known.