Make the e2e write phase work live across all five clients

Running the matrix against a live gateway surfaced several issues:

- The write phase is now opt-in (-VerifyWrite, was -SkipWrite). It runs
  right after register so only a small event backlog precedes the write,
  and asserts the reliable OnWriteComplete signal (the written value is
  not echoed back by a provider-driven attribute like TestChangingInt, so
  the value compare is best-effort).
- Java was launched as bare "gradle", which .NET's Process.Start cannot
  exec (it is gradle.bat) — resolve the launcher and run it via cmd.exe.
- The Java client's MxEventStream queue capacity was 16, which overflows
  on any active session's backlog-replay burst; raised to 1024.
- The Rust stream-events CLI now renders the event family as the proto
  enum name, matching the protobuf-JSON the other four clients emit.

Update docs/GatewayTesting.md for the reworked write phase.

Verified live: the full five-client matrix passes with -VerifyWrite.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

clients/java/mxgateway-client/src/main/java/com/dohertylan/mxgateway/client/MxGatewayClient.java

@@ -260,7 +260,12 @@ public final class MxGatewayClient implements AutoCloseable {
      * @return an iterator-style stream of events
      */
     public MxEventStream streamEvents(StreamEventsRequest request) {
-        MxEventStream stream = new MxEventStream(16);
+        // The buffer must absorb the gateway's session-backlog replay burst,
+        // which arrives far faster than the iterator drains it. A small queue
+        // overflows on any moderately active session; 1024 covers a realistic
+        // backlog while still bounding memory and preserving overflow
+        // detection for a genuinely unbounded stream.
+        MxEventStream stream = new MxEventStream(1024);
         MxGatewayChannels.withStreamDeadline(rawAsyncStub(), options).streamEvents(request, stream.observer());
         return stream;
     }