dankclaw Qwen3.8: batch 8192/ubatch 256 - fixes MTP collapse + premature-EOS

Found: large ubatch (512) collapses MTP acceptance to ~25% (gen ~9 t/s);
ubatch 128 with batch>=2048 triggers premature instant-EOS under 2 concurrent
slots.  ubatch 256 = ~30 t/s gen, concurrent-safe, ~330 t/s prefill.

scripts/frank-default-large.env

@@ -37,10 +37,13 @@ N_GPU_LAYERS=all
 # mixed types silently disable FA (long-context decode collapses).
 CACHE_TYPE_K=q8_0
 CACHE_TYPE_V=q8_0
-# Batch sizing (start conservative; tune after verifying memory):
-# ubatch is the compute-graph driver; larger = more VRAM, faster prefill.
-BATCH_SIZE=2048
-UBATCH_SIZE=512
+# Batch sizing (verified 2026-08-14 on Qwen3.8 UD-Q8_K_XL):
+#   ubatch 128 with batch>=2048 -> premature-EOS under concurrency (unsafe).
+#   ubatch 512 -> MTP acceptance collapses (~25%) -> gen ~9 t/s (slow).
+#   ubatch 256 -> ~30 t/s gen, concurrent-safe, ~330 t/s prefill.  WINNER.
+# batch 8192 keeps single-shot prefill flexibility; ubatch drives the graph.
+BATCH_SIZE=8192
+UBATCH_SIZE=256
 # Sampling settings — per Qwen3.8-27B model card thinking mode.
 # Thinking: temp=1.0, top_p=0.95, top_k=20, min_p=0. Clients may override.
 # WARNING: do not name this variable TEMP. The ROCm HIP runtime reads the