Keep the export venv off tmpfs

`mktemp -d` lands in `/tmp`, which on this and most current Linux
distributions is a tmpfs — memory, not disk, sized at half of RAM. The
venv this script builds installs torch into it, several gigabytes, and
the failure mode is not subtle:

    error: Failed to install: torch-2.13.0-...whl
      Caused by: No space left on device (os error 28)

on a machine with 102 GB free on the filesystem holding `/var/tmp`. The
quieter version of the same bug is worse: when it does fit, it evicts
whatever the user had in page cache to make room.

`${TMPDIR:-/var/tmp}` respects an explicit TMPDIR and otherwise picks the
disk-backed directory, which is what a multi-gigabyte throwaway wants.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tools/export-seg-model.sh

@@ -28,7 +28,12 @@ OUT="${REPO}/models/segment"
 
 MODEL="${1:-yolo26n-seg}"
 IMGSZ="${2:-640}"
-WORK="$(mktemp -d)"
+# Not `mktemp -d`: the default TMPDIR is `/tmp`, which on most current Linux
+# distributions is a tmpfs — RAM, sized at half of physical memory. The venv
+# below pulls torch, several gigabytes of it, and installing that into RAM
+# either evicts the user's page cache or fails outright with ENOSPC on a
+# machine that has hundreds of gigabytes of actual disk free.
+WORK="$(mktemp -d -p "${TMPDIR:-/var/tmp}")"
 trap 'rm -rf "${WORK}"' EXIT
 
 echo "==> exporting ${MODEL} at imgsz=${IMGSZ} in ${WORK}"