mlxrunner: Release freed KV buffers during speculative decode

The decode loop releases MLX's pool of freed buffers every 256 generated
tokens, which is also how often the KV cache grows and drops its previous,
smaller buffers. The check fires only when the token count lands exactly on
a multiple of 256. Speculative decoding emits several tokens per round, so
most rounds step over the boundary and the pool is never released. Each
growth at a long context leaves several GB of buffers that no later
allocation can reuse, so the runner's footprint keeps climbing over a long
generation until the system runs out of memory.

We now release the pool whenever a round crosses a multiple of 256 tokens,
which is what a single-token round already did. With qwen3.8:27b-mlx at a
98k-token context on a 128 GB machine, a long speculative generation
previously grew the runner past 90 GB and panicked the kernel; it now stays
flat at 30 GB.

mlxrunner/pipeline.go

@@ -278,7 +278,9 @@ func (r *Runner) decode(ctx context.Context, request Request, session *cacheSess
 	now := time.Now()
 
 	// Release MLX's cached free buffers every clearCacheInterval tokens so the
-	// allocator's pool does not grow unbounded over a long generation.
+	// allocator's pool does not grow unbounded over a long generation. A
+	// speculative round emits several tokens at once, so the clear fires on
+	// crossing a multiple of the interval, not on landing exactly on one.
 	const clearCacheInterval = 256
 
 	generated := 0
@@ -289,6 +291,7 @@ func (r *Runner) decode(ctx context.Context, request Request, session *cacheSess
 
 		var done bool
 		var err error
+		before := generated
 		mlx.Scoped(func() {
 			var results []sampler.Result
 			results, err = d.next(request.Options.NumPredict - generated)
@@ -348,7 +351,7 @@ func (r *Runner) decode(ctx context.Context, request Request, session *cacheSess
 			break
 		}
 
-		if generated%clearCacheInterval == 0 {
+		if generated/clearCacheInterval != before/clearCacheInterval {
 			mlx.ClearCache()
 		}
 	}