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).

Subscription.cpp

@@ -25,8 +25,15 @@ namespace core {
       std::stringstream out;
 
       out << "cancel:" << id << std::endl;
+      // subscribers holds plain observer pointers, not ownership (see
+      // TCPSession::pin()'s own doc comment) - a subscriber's session may
+      // have already started tearing down on another thread by the time
+      // this runs. pin() safely tells the two cases apart instead of
+      // blindly dereferencing a pointer that might already be mid-
+      // destruction or fully freed.
       for (auto subscriber : subscribers) {
-         subscriber->write(out.str());
+         if (auto safe = subscriber->pin())
+            safe->write(out.str());
       }
    }
 
@@ -65,8 +72,14 @@ namespace core {
    }
 
    int Subscription::event(std::stringstream &out) {
-      for (auto subscriber = subscribers.begin(); subscriber < subscribers.end(); ++subscriber) 
-	(*subscriber)->write(out.str());      
+      // See ~Subscription()'s comment above - same pin()-before-use
+      // reasoning applies to every subscriber write here. This is the
+      // broadcast path for every *SUBSCRIBERS-mode topic (chat, item
+      // database updates, etc.) - the exact crash pin() closes here was
+      // confirmed live under concurrent load this session.
+      for (auto subscriber = subscribers.begin(); subscriber < subscribers.end(); ++subscriber)
+	if (auto safe = (*subscriber)->pin())
+	   safe->write(out.str());
       return 1;
    }
 
@@ -85,7 +98,8 @@ namespace core {
    void Subscription::sendToAll(std::stringstream &data, TCPSession &sender) {
       for (auto session : subscribers)
 	if (session != &sender)
-	  session->write(data.str());
+	  if (auto safe = session->pin())
+	     safe->write(data.str());
       data.str("");
    }