S15.4, CPU half: XFeat runs in ~400 ms per frame on the tablet

tools/onnx-probe-on-device.sh cross-builds dr-segment's onnx_probe
without the embedded segmentation model, pushes it with a model to the
attached device and times two runs. The 768×1024 XFeat export takes
~400 ms on the reference tablet's NEON cores against ~300 ms on the
desktop, with identical output ranges — inside NFR-MRG-1's 1 s per frame.
The blend half of S15.4 waits for a chunked blend to exist.

tools/onnx-probe-on-device.sh

@@ -0,0 +1,60 @@
+#!/usr/bin/env bash
+# Time an ONNX model under tract on a connected Android device.
+#
+#   ./tools/onnx-probe-on-device.sh models/keypoints/xfeat-1024.onnx [1x1x768x1024]
+#
+# ## Why this exists
+#
+# Every model the app runs on the CPU — faces, masks, keypoints — runs under
+# tract on NEON when it runs on the tablet, and the desktop figure says
+# nothing about that: twenty cores of AVX2 against a tablet's big.LITTLE
+# cluster is not a ratio anyone should guess (faces.md §9 measured 2×
+# on one workload and would have guessed wrong). NFR-MRG-1 puts a per-frame
+# budget on keypoint detection *on the tablet*, and this is how the number
+# is measured rather than assumed.
+#
+# Cross-builds `dr-segment`'s `onnx_probe` example — without the embedded
+# segmentation model, which the probe does not need and which is 11 MB the
+# push would otherwise carry — pushes it with the model to `/data/local/tmp`,
+# runs it twice (tract's first run pays for planning) and removes both.
+#
+# Not part of CI, which has no device attached.
+set -euo pipefail
+
+TARGET=aarch64-linux-android
+API=26
+DEST=/data/local/tmp
+
+MODEL="${1:?usage: onnx-probe-on-device.sh <model.onnx> [NxCxHxW]}"
+SHAPE="${2:-1x1x768x1024}"
+
+ndk="${ANDROID_NDK_HOME:-}"
+if [[ -z "$ndk" ]]; then
+    ndk=$(find "${ANDROID_HOME:-$HOME/Android/Sdk}/ndk" -maxdepth 1 -mindepth 1 -type d 2>/dev/null |
+        sort -V | tail -1)
+fi
+clang="$ndk/toolchains/llvm/prebuilt/linux-x86_64/bin/$TARGET$API-clang"
+[[ -x "$clang" ]] || {
+    echo "no $TARGET$API-clang in $ndk — set ANDROID_NDK_HOME" >&2
+    exit 1
+}
+rustup target list --installed | grep -qx "$TARGET" || rustup target add "$TARGET"
+
+adb devices | grep -qw device || {
+    echo "no device attached — plug the tablet in and enable USB debugging" >&2
+    exit 1
+}
+
+echo "building onnx_probe for $TARGET…"
+CARGO_TARGET_AARCH64_LINUX_ANDROID_LINKER="$clang" \
+CC_aarch64_linux_android="$clang" \
+    cargo build -q -p dr-segment --example onnx_probe --release --target "$TARGET" \
+        --no-default-features --features semantic
+binary="${CARGO_TARGET_DIR:-target}/$TARGET/release/examples/onnx_probe"
+
+echo "pushing $(basename "$binary") and $(basename "$MODEL")…"
+adb push "$binary" "$DEST/onnx_probe" >/dev/null
+adb push "$MODEL" "$DEST/probe.onnx" >/dev/null
+adb shell chmod 755 "$DEST/onnx_probe"
+adb shell "$DEST/onnx_probe" "$DEST/probe.onnx" "$SHAPE"
+adb shell rm -f "$DEST/onnx_probe" "$DEST/probe.onnx"