Add regression test for concurrent dub output reads

[atilaneves]

Summary

Protects the deadlock fix in #334 with an integration test that writes 1 MiB to stderr before completing stdout. A ten-second timeout makes sequential pipe reads fail instead of hanging CI. Runs in both the POSIX and Windows CI scripts.

fake dub describe → fill stderr → write stdout JSON
getDubInfo → drain both streams → parse JSON before timeout

Evidence

• Before Avoid deadlock by reading stdout and stderr concurrently. #334: the same test fails with getDubInfo timed out while draining stdout and stderr.
• After Avoid deadlock by reading stdout and stderr concurrently. #334: Concurrent dub output regression test passed, including on merged master.
• Full Linux build/ci.sh passes. Windows execution is left to CI.

Merge Danger

Door: two-way; test-only changes can be reverted.

Blast Radius: CI.