Let the segmentation export script pick its own input size

Comparing a larger model against a larger input size meant re-exporting at
resolutions other than the shipped 640, and the script only ever wrote that
one number. `IMGSZ` is now a second positional argument, defaulted to 640 so
every existing call is unchanged.

The experiment this was built for found bigger input a net loss on its own
merits — yolo26n-seg and yolo26s-seg at 1280 both lost track of large,
frame-filling subjects (a bus's box shrank and its score nearly halved)
in exchange for catching small or partially-occluded ones tiling already
handles. Nothing shipped from it, but the ability to re-run that comparison
is worth keeping.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tools/export-seg-model.sh

@@ -17,7 +17,7 @@
 # (see the workspace manifest for why that backend was chosen), and it is what
 # makes the tiling option in `semantic.rs` the only route to more resolution.
 #
-# `imgsz=640` square rather than a rectangle matched to 3:2: one graph has to
+# `imgsz` square rather than a rectangle matched to 3:2: one graph has to
 # serve portrait, landscape, square crops and panoramas. A landscape-shaped
 # graph trades letterbox waste on 3:2 for worse waste on everything else.
 set -euo pipefail
@@ -27,21 +27,23 @@ REPO="$(cd "${HERE}/.." && pwd)"
 OUT="${REPO}/core/dr-segment/models"
 
 MODEL="${1:-yolo26n-seg}"
+IMGSZ="${2:-640}"
 WORK="$(mktemp -d)"
 trap 'rm -rf "${WORK}"' EXIT
 
-echo "==> exporting ${MODEL} in ${WORK}"
+echo "==> exporting ${MODEL} at imgsz=${IMGSZ} in ${WORK}"
 cd "${WORK}"
 uv venv --python 3.12 venv
 VIRTUAL_ENV="${WORK}/venv" uv pip install ultralytics onnx onnxslim
 
-VIRTUAL_ENV="${WORK}/venv" "${WORK}/venv/bin/python" - "${MODEL}" <<'PY'
+VIRTUAL_ENV="${WORK}/venv" "${WORK}/venv/bin/python" - "${MODEL}" "${IMGSZ}" <<'PY'
 import sys, json
 from ultralytics import YOLO
 
 name = sys.argv[1]
+imgsz = int(sys.argv[2])
 m = YOLO(f"{name}.pt")
-path = m.export(format="onnx", opset=17, simplify=True, imgsz=640, dynamic=False)
+path = m.export(format="onnx", opset=17, simplify=True, imgsz=imgsz, dynamic=False)
 print("ONNX:", path)
 
 # The class names travel with the model rather than being retyped into Rust —