frank-primary: pin the expert host region to 120 GiB (--expert-offload-mem)

The host region was the ONE arena-plan term still autofitting per boot: with
no cap the planner binds it to MemAvailable AT BOOT minus PLE/process/api/
margin terms (boot 2026-10-05: 237.97 GiB -> 159.39 GiB usable; the same host
reads 75.4 GiB available while running). Demand is 105.48 GiB
(52.74/rank x 2 ranks, 23,040 swap-target experts); a boot where co-tenant
RAM squeezed the bound near that silently pins next-hottest experts VRAM-only
(pinned_extra warning, smaller swap region, no flag change). 120 GiB =
measured demand + 14% headroom on 246.1 GiB total. Semantics verified in the
shipped image: cap binds as the bound, exceedance only warns.

The GPU-side arena is unchanged (deterministic arithmetic over the same
constants): verify after boot that host reads '--expert-offload-mem' and
slots stay 7,762. Seam: red first (wiring + value), then green; full suite
368+1 green.

scripts/serve-frank-primary.sh

@@ -156,6 +156,7 @@ podman run -d --name "$FRANK_PRIMARY_CONTAINER" \
   --max-num-seqs "$FRANK_PRIMARY_SEQS" \
   ${FRANK_PRIMARY_MNBT:+--max-num-batched-tokens "$FRANK_PRIMARY_MNBT"} \
   ${FRANK_PRIMARY_PRECISE_MEMORY:+--expert-precise-memory} \
+  ${FRANK_PRIMARY_EXPERT_OFFLOAD_MEM:+--expert-offload-mem "$FRANK_PRIMARY_EXPERT_OFFLOAD_MEM"} \
   --gpu-memory-utilization "$FRANK_PRIMARY_UTIL" \
   --kv-cache-memory-bytes "$FRANK_PRIMARY_KV_BYTES" \
   --kv-cache-dtype "$FRANK_PRIMARY_KV_DTYPE" \