docs: final 256k verdict — boots at 4.03-4.5 GiB KV, band 34.1-36.5 straddles target

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.

serverAppConfigs-docs/log.md

@@ -96,6 +96,15 @@
     under 35). KV-exact-4.0 arm in flight for the last +3.5% slots.
     Correction recorded: the planner sizes the arena AFTER the KV override
     (the earlier "before" reading was wrong) — the arena is what remains.
+  - **Final 256k probe: the KV trim is inert.** KV-exact 4.0 GiB
+    boot-refused by 20 MB of accounting (3.98 available < 4.0 needed;
+    engine estimate at 3.98: max len 260,176); +32 MiB (`KV_BYTES=
+    4328521728`) boots — but the arena is **identical to the 4.5 GiB arm
+    (5,648 slots)**: the planner buckets, the 0.47 GiB went to margin.
+    Decode band **36.48/34.13/35.02** (acceptance 39.4%) — same 34.1-36.5
+    band, straddling the 35 target within noise. Final 256k config:
+    `MAXLEN=262144 KV_BYTES=4328521728`, prefetch off. The full-speed
+    alternative stays `MAXLEN=184224` at 3 GiB (37.4-40.6 band).
 
 ## 2026-09-28
 - **Dankclaw model cleanup** (disk was at 93%, 137 G free). Frank asked to