docs(frank-primary): RESOLVED - the ~40k corruption was marginal DDR5 at 4800, fixed at 4600

The prefix-caching OFF/ON experiment was confounded (both arms ran on
the marginal speed); the align-mode re-seed theory is unproven, not
disproven. Caching ships ON with zero gdn_hip faults under heavy
re-send traffic on stable RAM; knob stays commented. Corrects the
env's 'RAM RULED OUT' claim and adds the AGENTS.md lesson.

AGENTS.md

@@ -729,6 +729,17 @@ linked runbooks.
   the KV pool is worse than the OOM it prevents.
 - Host swap state is invisible to `deploy.sh` (no repo file) — check
   `swapon --show` before reasoning about headroom.
+- **Silent RAM corruption outranks every software theory.** frank-primary's
+  ~40k-token output corruption (2026-10-04/05, repetition/garbage in long
+  agent conversations) was marginal DDR5 at 4800 MT/s on four DIMMs, fixed
+  by 4600. Consumer non-ECC memory raises **no MCE** for a silent flip, and
+  a short in-band pattern check (60 s, 8 GiB of 246 GiB) is not a stability
+  proof — combined, those two instruments said "ruled out" while the RAM
+  was the cause. "4800 is the JEDEC floor on 5600-rated DIMMs" is a rating,
+  not a margin guarantee. When the fault is data-shaped (NaN states,
+  garbage tokens) and a software revert doesn't move it, price in the RAM
+  before the runtime — and remember a revert tested under the same marginal
+  hardware is itself confounded. See the runbook's 2026-10-05 section.
 
 **Runtime-specific guards**