docs(iommu): correct the identity-mode cost claim; pt is most of the fix, PerfOpt the last part

Reading patch v2 1/2 verbatim: amd_iommu_enable_perfopt() rehoming needs the
device ALREADY on the identity domain and switches ATS/PRI/PASID/GCR3 off via
the skip_caps path. So an iGPU in identity mode still pays a 1:1 page walk plus
those four capabilities until PerfOpt exists -- iommu=pt recovers most of the
gap, PerfOpt the last part. The first pass claimed pt alone already avoids the
cost; that was wrong and is retracted here and in the posted issue comment
(edit 23).

Also re-keys frank-npu.md, which carried the dead law 'iommu=pt is not a middle
ground' as its retirement rationale: identity mode + SVA is exactly the revival
path (amdxdna binds its own IOPT_TYPE_SVA domain), and the cost is single-digit
percent on this silicon class. Procedure step 2 now demands a local off-vs-pt
measurement instead of inheriting the vendor's 13-16%, whose slow-arm boot line
our record never identified.

AGENTS.md

@@ -1107,15 +1107,16 @@ linked runbooks.
   times (`RuntimeError: No HIP GPUs are available`).
 - SearXNG outbound traffic must stay on the Gluetun tunnel; do not move its
   ports off the gluetun service.
-- `amd_iommu=off` on Dankclaw (bought 13–16 % prefill for frank-dragoon,
-  2026-09-11) retires the XDNA2 NPU: the driver's SVA/PASID bind needs an
-  enabled IOMMU (`/dev/accel/accel0` fails `err=-19`). That trade is on an
-  expiring path: **AMD PerfOpt** (iommu tree 2026-09-24, Linux 7.4) stops
-  iGPU GPA→SPA translation while the IOMMU stays enabled for every other
-  device, so the next boot-line window is `iommu=pt` with the NPU usable —
-  measured only 2–4 % residual on a Strix Halo desktop (18–23 % on Strix
-  Point laptops). Not installable here yet: Ubuntu 26.04 caps at
-  `7.0.0-38`. Full research in
+- `amd_iommu=off` on Dankclaw (adopted 2026-09-11 on the vendor's 13–16 %
+  prefill figure, never measured here) retires the XDNA2 NPU: the driver's
+  SVA/PASID bind needs an enabled IOMMU (`/dev/accel/accel0` fails `err=-19`).
+  That trade is on an expiring path: **AMD PerfOpt** (iommu tree 2026-09-24,
+  Linux 7.4) makes the IOMMU skip GPA→SPA translation for the *privileged iGPU*
+  while staying fully armed for every other device — and it arms only when the
+  iGPU is already in the identity domain, so the next boot-line window is
+  `iommu=pt` with the NPU usable, not `off`. PerfOpt buys 2–4 % back on a Strix
+  Halo desktop (18–23 % on Strix Point laptops); it is not installable here
+  yet (Ubuntu 26.04 caps at `7.0.0-38`). Research in
   `serverAppConfigs-docs/LLM/dankclaw-iommu-translation-off.md`.
 - Rootless Quadlet containers run with `--replace --rm`: `podman logs` covers
   only the CURRENT instance; history across recreations lives in