Add the int8 detectors for the Hexagon, calibrated on real photographs

tools/quantise-models.sh writes the QDQ form QNN's HTP backend takes
whole: opset 17, per-channel int8 weights, uint8 activations, ranges
from running the f32 graph over photographs fed exactly as the app
feeds them. The calibration is strided, four images at a time, because
every ONNX Runtime calibrator holds each image's whole set of
activations until it folds them — a gigabyte an image on the 10g
detector, and an OOM kill with no message when folded once at the end.

Release-time, never on the device (docs/inference.md §5): it needs
real photographs and a person reading the recall measurement that
gates whether each file is offered.

tools/quantise-models.sh

@@ -0,0 +1,35 @@
+#!/usr/bin/env bash
+# Produce the int8 form of a model for the Hexagon (docs/inference.md §5).
+#
+#   ./tools/quantise-models.sh PHOTO_DIR MODEL.onnx [MODEL.onnx ...]
+#
+# Writes `MODEL.int8.onnx` beside each input: a QDQ graph, per-channel int8
+# weights, uint8 activations — the form QNN's HTP backend takes whole. The
+# activations' ranges come from running the f32 model over the photographs in
+# PHOTO_DIR, fed exactly as the app feeds them (letterboxed to the model's
+# input, the detector's `(x - 127.5) / 128` normalisation), which is why
+# this is a release-time step and not something the device does: it needs
+# real photographs and, after it, a person reading §10 M2's numbers.
+#
+# The SCRFD and ArcFace exports are opset 11; per-channel QDQ needs 13, so a
+# model below 13 is first upgraded to 17. That changes only the graph's
+# spelling, not a weight — and it is what `tools/fix-face-model-shapes.sh`
+# will do to the canonical files in the same model release.
+#
+# A venv per run, like fix-face-model-shapes.sh: the tools are not a build
+# input and nothing in the tree should have them on its path.
+set -euo pipefail
+if [ "$#" -lt 2 ]; then
+    sed -n '2,20p' "$0" >&2
+    exit 2
+fi
+PHOTOS="$1"; shift
+[ -d "${PHOTOS}" ] || { echo "no such directory: ${PHOTOS}" >&2; exit 1; }
+
+WORK="$(mktemp -d -p /var/tmp quantise-models.XXXXXX)"
+trap 'rm -rf "${WORK}"' EXIT
+echo "==> venv in ${WORK}"
+uv venv --python 3.12 "${WORK}/venv" >/dev/null
+VIRTUAL_ENV="${WORK}/venv" uv pip install --quiet onnx onnxruntime pillow numpy sympy
+
+exec "${WORK}/venv/bin/python" "$(dirname "$0")/quantise-models.py" "${PHOTOS}" "$@"