bugfix: resolve performance issue with steamshim

[pollend]

This pull request introduces a significant refactor and improvement to the Steam networking (SDR) packet handling system, transitioning from synchronous, request-based message polling to an event-driven, buffered architecture. The main goals are to improve packet delivery efficiency, reduce blocking, and ensure robust cleanup of resources. The changes affect both the game-side networking code (net.c) and the SteamShim IPC layer (parent.cpp, child.cpp, mod_steam.h).

The most important changes are:

Networking Layer Refactor and Buffering:

• Replaces synchronous packet polling with an event-driven, buffered system for SDR packets. Incoming Steam messages are now buffered per connection slot and delivered asynchronously to the game, improving performance and reliability (net.c). [1] [2]
• Adds robust cleanup of buffered packets and slot management on disconnect and shutdown to prevent memory leaks (net.c). [1] [2]

SteamShim Event and Message Handling:

• Implements background draining of all tracked Steam connections in the SteamShim child process, pushing received messages as events to the parent process. The parent now processes these events via a non-blocking, buffered dispatch loop, decoupling network polling from game logic (child.cpp, parent.cpp). [1] [2] [3] [4]
• Refactors the parent dispatch logic to use a shared accumulation buffer (dispatchBuf) for efficient, non-blocking packet processing, and ensures correct synchronization and memory management (parent.cpp).

API and Interface Updates:

• Updates the SteamShim interface to expose the current sync index and ensure all new functions are properly declared and accessible (mod_steam.h). [1] [2] [3] [4]

These changes collectively modernize the SDR networking path, making it more robust, less blocking, and easier to maintain.