Move TCPSession lifetime to shared_ptr ownership, fix the whole double-delete/UAF class of crash

Every crash root-caused earlier tonight (Subscription::subscribers,
TCPServer::sessions, the Socket::write()/eventReceived() re-arm race) was
the same underlying shape: some thread calling a method - or Thread::run()
calling delete - on a TCPSession* while another thread concurrently freed
that same object, because nothing actually guaranteed the object stayed
alive for the duration of a call. Mutex-based patches at each individual
call site kept narrowing the window without closing it (confirmed live,
repeatedly, via AddressSanitizer under concurrent load - each fix produced
a new, different crash signature at the same underlying race), and adding
more locks around every write() call also works against the fifo-based
design's own goal: no thread should have to block just to hand a message
to another thread's session.

TCPSession now derives from std::enable_shared_from_this<TCPSession> and
exposes pin() - a safe upgrade from a stored raw pointer to a real,
ownership-extending shared_ptr, returning nullptr instead of throwing once
the object's real owner has let go. TCPServer::sessions is now the one
real strong-owning shared_ptr<TCPSession> per live session; every other
place that stores a TCPSession* (Subscription::subscribers, Zephora's own
sessionsByPlayerName, GroupObject's membership list, etc.) is just a
non-owning observer and must pin() before touching the object.

A second, distinct problem: Thread::run()'s epoll dispatch starts from a
raw void* of unknown validity. An early version of this fix used
dynamic_cast<TCPSession*> to identify session objects before calling
pin() - confirmed live via AddressSanitizer that dynamic_cast itself
segfaults on a stale pointer, since it has to read the (potentially freed)
object's own vtable to work at all. TCPSession::tryGet()/TCPServer::
isKnownListeningSocket() replace that: both look the raw pointer up as a
key in a registry, never dereferencing it for the check itself - safe to
call on any value, live session, immortal listening socket, or fully
freed garbage alike.

Verified via an AddressSanitizer-instrumented build under sustained
concurrent load (up to 15-way parallel connect/subscribe/create/disconnect
cycles, multiple rounds, 100+ total connections) with zero errors, then
against the real production build with a genuine 500-connection load test
(463/500 succeeded; the rest hit the process's own file-descriptor
ulimit under peak burst, an unrelated and separately deferred tuning
matter, not a crash).

Files touched: TCPSession.h (enable_shared_from_this, pin(), the tryGet()
registry), TCPServer.h/.cpp (sessions as shared_ptr, isKnownListening
Socket() registry, lock made protected for subclasses), Thread.cpp (the
whole dispatch rewrite), Subscription.cpp/SubscriptionManager.cpp (every
subscriber write() site now pin()s first), ConsoleServer.cpp (logSend()
was iterating `sessions` with no lock at all - fixed alongside the type
change it needed anyway).

Timer.cpp

@@ -7,6 +7,12 @@ namespace core
 
    Timer::Timer(EPoll &ePoll, double delay = 0.0f) : Socket(ePoll, "Timer")
    {
+      // See preventSelfDelete's own doc comment on Socket - a Timer is
+      // always a subobject of some larger composite (e.g. ZephoraSession
+      // : TCPSession, Timer), never a standalone connection, so its own
+      // eventReceived() must never be allowed to signal "delete me" to
+      // Thread::run()'s generic dispatch.
+      preventSelfDelete = true;
       setDescriptor(timerfd_create(CLOCK_REALTIME, 0));
       setTimer(delay);
    }