frank-default-large: threads-batch=16 confirmed optimal + decode benchmark results

Decode sweep (TB=8,16,32, 8K+ prompt, 256 gen tokens):
  TB=8:  3.3 tok/s (76.6s)
  TB=16: 3.4 tok/s (74.8s)
  TB=32: 3.4 tok/s (75.0s)

Decode is flat; TB=16 wins on prefill (5.1% faster) with zero decode tradeoff.

Also:
- Fix :: syntax error in serve-frank-large.sh
- Document systemctl disable destroys unit file; use mask/unmask instead

scripts/serve-frank-large.sh

@@ -49,7 +49,7 @@ fi
 : "${BATCH_SIZE:=4096}"
 : "${UBATCH_SIZE:=4096}"
 : "${NUM_THREADS:=16}"
-: "${THREAD_BATCH:=32}"
+: "${THREAD_BATCH:=16}"
 : "${CACHE_TYPE_K:=f16}"
 : "${CACHE_TYPE_V:=f16}"
 : "${CTX_CHECKPOINTS:=16}"