fix(frank-dragoon): mount the distro XRT at the third prefix the NPU child searches

The lane's child dlopens libxrt_core.so.2 from /opt/xilinx/lib/<multiarch>/
as well; our mounts filled only /opt/xilinx/xrt/lib and the install path.
Since the host's libxrt2 was reconfigured 2026-10-20 the child dies naming
the missing prefix ('No such library
/opt/xilinx/lib/x86_64-linux-gnu/libxrt_core.so.2') and the engine
supervisor takes the WHOLE container down on a dead lane — measured
2026-10-21: frank-dragoon boot-looped 88 times with chat dead while the
GPU lines looked healthy.

Each XRT library now mounts THREE times from the same real file (never a
symlink); the multiarch name derives from FRANK_DRAGOON_XRT_DIR's basename
so the bats fixture proves the derivation. Proven end-to-end before commit:
production-shaped scratch boot + the three extra mounts answers the full
boot ledger, npu_api buckets u0..u9, and /health ok at the 262144 ceiling.

A dead lane is not survivable on this engine — arm it only with all three
prefixes (page law updated).

scripts/serve-frank-dragoon.sh

@@ -136,10 +136,17 @@ fi
 #      The engine re-checks size+sha256 against that catalog at boot; it cannot
 #      fetch anything here because /models is read-only and HALOGEN_DOWNLOAD
 #      stays unset — no outbound connection at boot),
-#   2. the distro XRT's three libraries, each mounted twice (where the image
-#      looks for XRT, and at its own path — the libraries find each other there;
-#      an /opt/xilinx/xrt of symlinks into the system dir dangles inside the
-#      container, which the vendor calls out explicitly),
+#   2. the distro XRT's three libraries, each mounted THREE times: where the
+#      image looks for XRT (/opt/xilinx/xrt/lib), at their own path (the
+#      libraries find each other there), and at /opt/xilinx/lib/<multiarch>
+#      — the NPU child searches that third prefix too, and since the host's
+#      libxrt2 was reconfigured 2026-10-20 it is the ONLY prefix the child
+#      names: omitting it crashes the lane with `No such library
+#      '/opt/xilinx/lib/x86_64-linux-gnu/libxrt_core.so.2'` and the engine
+#      supervisor takes the dead lane down on the whole container (measured
+#      2026-10-21: 88-restart boot loop). Real files only — an /opt/xilinx/xrt
+#      of symlinks into the system dir dangles inside the container, which the
+#      vendor calls out explicitly),
 #   3. the GPU fabric clock HELD: on Strix Halo the iGPU and the NPU co-running
 #      hang the machine while FCLK changes speed, and the engine refuses to
 #      start when it moves — it cannot read host sysfs from inside the
@@ -190,7 +197,9 @@ if [ -n "$NPU_MODELS" ]; then
             exit 1
         fi
         REAL="$(readlink -f "$XRT_DIR/$L")"
-        NPU_ARGS+=(-v "$REAL:/opt/xilinx/xrt/lib/$L:ro" -v "$REAL:$XRT_DIR/$L:ro")
+        MULTIARCH="${XRT_DIR##*/}"
+        NPU_ARGS+=(-v "$REAL:/opt/xilinx/xrt/lib/$L:ro" -v "$REAL:$XRT_DIR/$L:ro"
+                   -v "$REAL:/opt/xilinx/lib/$MULTIARCH/$L:ro")
     done
     HELD=0
     for D in "$SYSFS"/class/drm/card*/device; do