Add configurable gameplay rates

[samburgess]

Summary

• add a helper API for configuring normal and fast-forward gameplay rates
• restore the configured active rate after pausing and when fast-forward changes
• keep audio consumption synchronized with the core using pitch-preserving time scaling
• convert core PCM through a mixer before the time-pitch unit so integer core formats are accepted
• disable display synchronization whenever the selected rate is faster than real time

The API preserves the current behavior by default: normal gameplay remains 1× and fast-forward remains 5× until the host configures different values.

Testing

• built the OpenEmu workspace with the OpenEmu scheme in Debug configuration
• verified both XPC and thread-mode implementations compile through the shared OEGameCoreHelper interface
• launched and played Pokémon FireRed with the mGBA core without an audio-graph failure

The OpenEmuKit test target currently fails to resolve OpenEmuKitPrivate under Xcode 26 before running tests; this is unrelated to these changes.