mlxrunner: raise the MTP pending-flush cap to 256 tokens

Per-token cost of the batched head forward keeps falling until the
flush is large enough to reach the fastest kernels: NAX matmul tiles
for dense heads, and the segmented gather path for MoE heads, which
needs tokens*topK/experts >= 4. Measured across the qwen3.6 heads,
256 is the smallest cap past every threshold and within a few percent
of each head's per-token floor. The cost is bounded: up to 2.5 MiB of
pinned hiddens per request and a flush stall under one decode step.

x/mlxrunner/mtp.go

@@ -11,7 +11,7 @@ import (
 // mtpPendingFlushTokens caps how many committed look-ahead tokens wait in the
 // pending buffer before a batched flush, bounding the pinned hidden states
 // regardless of what else triggers a flush.
-const mtpPendingFlushTokens = 32
+const mtpPendingFlushTokens = 256
 
 // mtpDrafter drafts with a model's multi-token-prediction head. Constructed
 // at load, it fixes the trie keys' draft look-ahead for the model's lifetime