docs: add MVP end-to-end test guide

Add a dedicated manual test guide for the MVP proof. The guide keeps the
operator flow concrete: relay startup, certificate handoff, Linux gateway
startup, elevated Windows client startup, expected log signals, acceptance
checks, troubleshooting, cleanup, and the information to report back.

This is intentionally scoped to the manual MVP. It documents that installer,
GUI, production certificates, authentication, and end-to-end payload encryption
are outside this proof.

Test Plan:
- git diff --check
- reviewed TESTING.md for command flow and expected log signals

Refs: PLAN.md Phase 1 success criteria

README.md

@@ -117,6 +117,8 @@ Public relay binary and relay-owned room state:
 cargo check --workspace
 ```
 
+For the manual MVP end-to-end proof, see [TESTING.md](TESTING.md).
+
 ## Relay
 
 ```bash