frank-primary: prefix caching back ON — the OFF experiment did not fix the corruption

Frank: still corrupted at ~40k with --no-enable-prefix-caching deployed
and mechanically verified (zero gdn_hip faults in the container serving
that window, coherent 40,771-token boundary probe). The recurrence is
FAULT-FREE, so the align-mode page re-seed is not the (only) mechanism
and the mitigation bought whole-history re-prefill per turn for nothing.

Revert: knob FRANK_PRIMARY_NO_PREFIX_CACHING commented out -> the serve
branch renders --enable-prefix-caching (the shipped contract). The knob,
the branch and the seam stay documented for a future fixed fork build.

Open hypotheses recorded in the runbook: re-fed poisoned transcript (the
same conversation cannot distinguish fixed from unfixed), or a second
fault-free long-context path — fp8 KV at depth is the next suspect. Next
diagnostic: fresh-conversation reproduction, then a KV-dtype A/B.

scripts/serve-frank-primary.sh

@@ -137,13 +137,14 @@ fi
 # available" while device_count() says 2). --ipc=host for the engine's shared
 # memory; memlock unbounded for the pinned host slabs.
 # The entrypoint already execs `vllm serve "$@"`, so no `serve` here.
-# Prefix caching: the fork's QFN mamba align path corrupts GDN SSM state
-# at ~40k tokens (2026-10-04, 48 gdn_hip state faults, one request — the
-# evidence block is in frank-primary.env). FRANK_PRIMARY_NO_PREFIX_CACHING
-# flips to the fork-named mitigation (--no-enable-prefix-caching); empty
-# keeps the historical flag. NO comment lines inside the backslash chain
-# below — a comment there terminates the continuation and the next line
-# runs as its own command (the 2026-10-05 exit-127 crash loop).
+# Prefix caching: ON — the shipped contract. FRANK_PRIMARY_NO_PREFIX_-
+# CACHING=1 flips to the fork-named mitigation (--no-enable-prefix-caching)
+# for the QFN align-mode GDN state fault; it was tried 2026-10-05 and
+# REVERTED the same day (live conversations still degraded at ~40k with
+# caching off and zero faults — see the evidence block in
+# frank-primary.env). NO comment lines inside the backslash chain below —
+# a comment there terminates the continuation and the next line runs as
+# its own command (the 2026-10-05 exit-127 crash loop).
 # The ${VAR:-word} / ${VAR:+word} double-expansion for the two directions is
 # AMBIGUOUS bash (${VAR:--flag} parses as a substring offset when set —
 # measured 2026-10-05: rendered "--no-enable-prefix-caching1"), so the flag