The KV trim is inert: arena identical (5,648 slots) at 4.5 and 4.03 GiB (planner buckets; the trim went to margin). Final 256k config MAXLEN=262144 KV_BYTES=4328521728 prefetch off; full-speed alternative MAXLEN=184224 at 3 GiB.
Measured campaign on the ad hoc qwen-fp8 container (tcclaviger RDNA4 fork, 2x R9700, TP=2): telemetry (with_stack=false) measured the prefetcher at 36% precision — ~half of gather traffic wasted; deleting --expert-prefetch: 26.8 -> 40.57/37.44/37.57 tok/s. Prefetch-on shape survived every other challenger (gather geometry, spec depth, TunableOp [corrupts numerics], MNBT, concurrency, ngram [PLE fork bug]). 256k ctx boots at 4.5 GiB KV (36.65/34.21/34.77, arena -14%). New runbook + index links + log entry.
Frank