feat(frank-dragoon): run the XDNA2 NPU lane beside the flash model

halogen-flash-server already ships an NPU path on the pin we run: for each
model named in HALOGEN_NPU_MODELS it loads the vendor's precompiled AIE
programs (XDNA2 executes only AIE-compiled graphs) and answers them on
/v1/embeddings inside the SAME container behind port 7770. That gives the
fleet an in-house embedding endpoint without touching the iGPU trunk.

The lane is opt-in and one line wide: HALOGEN_NPU_MODELS blank is the
pre-lane endpoint byte for byte (no device, no mounts, no engine env).

The three host preconditions the vendor names are gated in the serve script
and mirrored in the deployer (the 2026-10-01 twin law), each refused by name:
  - the staged model files. /models is read-only and HALOGEN_DOWNLOAD stays
    unset so the container opens no outbound connection at boot, so
    scripts/download-frank-dragoon-npu.sh stages them from the pinned
    revision with the catalog's own byte counts and sha256s (the engine
    re-checks both at boot);
  - the distro XRT's libxrt_coreutil/libxrt_core/libxrt_driver_xdna, each
    mounted twice — where the image looks for XRT and at its own path —
    because those libraries find each other where they were installed and
    an /opt/xilinx/xrt of symlinks dangles inside the container;
  - the GPU fabric clock HELD. On Strix Halo the iGPU and the NPU co-running
    hang the machine (and corrupt NPU results) while FCLK changes speed, and
    halogen refuses to start in that state; it cannot read host sysfs from
    inside, so the hold is proven host-side. dankclaw/host/ carries the
    vendor's halogen-fabric-clock{,.service} verbatim at tag v0.16.2 (sha256
    a3a4f7f4e0142e71… / 5f9f1cc4c85a1c02…); the deployer installs, enables
    and gates it only while the lane is named, and releases the clock plus
    drops the boot hook when it is not.

After health, the deployer's lane gate is a real embedding through
/v1/embeddings — a 200 on /health proves nothing about the NPU programs.

Planner: dankclaw/host/halogen-fabric-clock* re-dispatches frank-dragoon
(the deployer installs it); dankclaw/host/grub and the fetch script plan
nothing.

Seam: 11 new tests — the off arm really renders nothing lane-specific, the
on arm renders the device/mounts/env, every missing precondition refuses by
name, the deployer twins and the host unit shape are pinned.

Co-Authored-By: omp <omp@dankclaw>

scripts/frank-dragoon.env

@@ -80,3 +80,33 @@ HALOGEN_TOP_K=20
 # Weights stay pinned: the engine pins by default and only the retired w4b build
 # honored HALOGEN_FLASH_PIN_TRUNK=0, which this payload rejects.
 HALOGEN_WEIGHTS_LOCK=1
+
+# --- NPU lane (XDNA2) -------------------------------------------------------
+# HALOGEN_NPU_MODELS is the whole switch: EMPTY or unset = the endpoint exactly
+# as it ran before this lane existed (no accel device, no XRT mount, no
+# fabric-clock dependency, no /v1/embeddings). Rolling the lane back is blanking
+# this one line and re-deploying — nothing else moves.
+# Names are the vendor catalog's folder names (`/opt/halogen/npu/models.txt`
+# inside the image). The engine cannot fetch them here: /models is read-only and
+# HALOGEN_DOWNLOAD stays unset so the container opens no outbound connection at
+# boot, so scripts/download-frank-dragoon-npu.sh must stage them into
+# $FRANK_DRAGOON_NPU_DIR first (the serve gate and the deployer both refuse by
+# name when a named model is not staged).
+# Cost: ~1.5 GiB extra pinned host RAM per named model, and a hard dependency on
+# the host holding the GPU fabric clock — on Strix Halo the iGPU and the NPU
+# together hang the machine (and corrupt NPU results) while FCLK changes speed,
+# so halogen refuses to start with this variable set unless the clock is held
+# (dankclaw/host/halogen-fabric-clock; the deployer installs and gates it).
+HALOGEN_NPU_MODELS=qwen3-embedding-0.6b
+FRANK_DRAGOON_NPU_DIR=/home/dank/models/halogen-npu
+# Node paths and the XRT directory are variables so the bats seam can drive
+# these gates on a box with no NPU. Ubuntu's distro XRT (libxrt2 + libxrt-npu2)
+# is mounted FILE BY FILE and TWICE EACH — once where the image looks for XRT,
+# once at its own path, because those libraries find each other at their install
+# path; the vendor forbids mounting an /opt/xilinx/xrt of symlinks into the
+# system dir, which dangle inside the container.
+FRANK_DRAGOON_DEV_ACCEL=/dev/accel/accel0
+FRANK_DRAGOON_XRT_DIR=/usr/lib/x86_64-linux-gnu
+# sysfs root the fabric-clock preflight reads (same knob the vendor's
+# halogen-fabric-clock status reads; the seam points it at a fixture).
+FRANK_DRAGOON_SYSFS=/sys