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).

tests/frank_dragoon_unit.bats

@@ -430,9 +430,12 @@ npu_fixture() {
     # the stock root:render 0660 mode, and the serve gate checks exactly that.
     touch "$TEST_TMP/accel0"
     chmod 666 "$TEST_TMP/accel0"
-    mkdir -p "$TEST_TMP/xrt"
+    # basename carries a multiarch shape: the third mount prefix is derived
+    # from the XRT dir's basename (the NPU child searches
+    # /opt/xilinx/lib/<multiarch>/, measured 2026-10-21).
+    mkdir -p "$TEST_TMP/x86_64-linux-gnu"
     for l in libxrt_coreutil.so.2 libxrt_core.so.2 libxrt_driver_xdna.so.2; do
-        touch "$TEST_TMP/xrt/$l"
+        touch "$TEST_TMP/x86_64-linux-gnu/$l"
     done
     mkdir -p "$TEST_TMP/npu/qwen3-embedding-0.6b/devices"
     mkdir -p "$TEST_TMP/sys/class/drm/card1/device"
@@ -442,7 +445,7 @@ npu_fixture() {
         "HALOGEN_NPU_MODELS=qwen3-embedding-0.6b"
         "FRANK_DRAGOON_NPU_DIR=$TEST_TMP/npu"
         "FRANK_DRAGOON_DEV_ACCEL=$TEST_TMP/accel0"
-        "FRANK_DRAGOON_XRT_DIR=$TEST_TMP/xrt"
+        "FRANK_DRAGOON_XRT_DIR=$TEST_TMP/x86_64-linux-gnu"
         "FRANK_DRAGOON_SYSFS=$TEST_TMP/sys"
     )
 }
@@ -472,11 +475,15 @@ $(cat "$TEST_TMP/serve.log")"
     ! grep -q 'accel0' "$TEST_TMP/serve.log"
 }
 
-@test "the lane arms the accel node, the staged model dir and each XRT library twice" {
-    # Each library mounts at the image's XRT path AND at its own path: the
-    # distro libraries find each other where they were installed, and an
-    # /opt/xilinx/xrt of symlinks into the system dir dangles inside the
-    # container (the vendor calls that out by name).
+@test "the lane arms the accel node, the staged model dir and each XRT library three times" {
+    # Each library mounts at the image's XRT path, at its own path (the distro
+    # libraries find each other where they were installed), and at
+    # /opt/xilinx/lib/<multiarch>/ — the prefix the NPU child searches and
+    # names when it is missing (`No such library
+    # '/opt/xilinx/lib/x86_64-linux-gnu/libxrt_core.so.2'`, measured
+    # 2026-10-21: the lane died and the supervisor took the whole container
+    # with it, 88-restart loop). An /opt/xilinx/xrt of symlinks into the
+    # system dir dangles inside the container (the vendor calls that out).
     podman_stub
     good_env
     npu_fixture
@@ -491,8 +498,9 @@ $(cat "$TEST_TMP/serve.log")"
     [[ "$output" != *":/models/npu"* ]]
     [[ "$output" == *"-e HALOGEN_NPU_MODELS=qwen3-embedding-0.6b"* ]]
     for l in libxrt_coreutil.so.2 libxrt_core.so.2 libxrt_driver_xdna.so.2; do
-        [[ "$output" == *"-v $TEST_TMP/xrt/$l:/opt/xilinx/xrt/lib/$l:ro"* ]]
-        [[ "$output" == *"-v $TEST_TMP/xrt/$l:$TEST_TMP/xrt/$l:ro"* ]]
+        [[ "$output" == *"-v $TEST_TMP/x86_64-linux-gnu/$l:/opt/xilinx/xrt/lib/$l:ro"* ]]
+        [[ "$output" == *"-v $TEST_TMP/x86_64-linux-gnu/$l:$TEST_TMP/x86_64-linux-gnu/$l:ro"* ]]
+        [[ "$output" == *"-v $TEST_TMP/x86_64-linux-gnu/$l:/opt/xilinx/lib/x86_64-linux-gnu/$l:ro"* ]]
     done
     # The Flash payload still rides the same container: the lane is additive.
     [[ "$output" == *"-e HALOGEN_CHECKPOINT=/models/qwen.hgn"* ]]
@@ -567,7 +575,7 @@ $(cat "$TEST_TMP/serve.log")"
     podman_stub
     good_env
     npu_fixture
-    rm "$TEST_TMP/xrt/libxrt_driver_xdna.so.2"
+    rm "$TEST_TMP/x86_64-linux-gnu/libxrt_driver_xdna.so.2"
     run_with_log
     [ "$status" -ne 0 ]
     [[ "$output" == *"libxrt_driver_xdna.so.2 missing"* ]]