multiprocess: recover from stale SysV shm + unix socket after crash

Linux QSharedMemory uses SysV segments that survive process crashes.
On the next launch, m_SharedMem.create() fails with AlreadyExists and
the constructor would either treat the corpse as a live primary
(blocking the user behind a ghost) or throw. The unix socket file is
similarly left behind and would make QLocalServer::listen() fail with
AddressInUseError if reclaim succeeded.

Now: probe the primary's listener via QLocalSocket::connectToServer
before believing the segment is alive. If the probe fails, detach and
re-create. If create then fails (another attached client keeps refcount
> 0), re-attach so the constructor returns in a usable secondary state
instead of silently no-op'ing every later sendMessage. Also call
QLocalServer::removeServer() before listen() to clear the orphaned
socket file when we did successfully reclaim.

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

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