test/auto-install.sh: auto-fetch + reassemble chunked ISO from ci-latest

Workflow now publishes ISO as 1900M chunks. Test harness needs to:
1. gh release download --pattern '*.iso.part-*'
2. cat parts back into single ISO
3. verify sha256 of all parts

If invoked with no ISO arg, auto-fetches from ci-latest release.
Falls back to local ISO path if given as arg (existing behavior).

Reassembled ISO lives at ~/veilor-iso/ci-latest/.

test/auto-install.sh

@@ -88,8 +88,27 @@ preflight() {
 
     ISO="${1:-}"
     if [[ -z $ISO ]]; then
-        echo "Usage: $0 <path-to-veilor-os.iso>" >&2
+        # Auto-fetch from ci-latest GH release if no path given. ISO is split
+        # into chunks (GH release 2 GiB asset cap). Reassemble before boot.
+        log "no ISO path given — fetching from gh release ci-latest"
+        local dl_dir="$HOME/veilor-iso/ci-latest"
+        mkdir -p "$dl_dir"
+        ( cd "$dl_dir" && rm -f *.part-* *.iso *.sha256 && \
+          gh release download ci-latest --repo veilor-org/veilor-os \
+            --pattern '*.iso.part-*' --pattern '*.parts.sha256' --clobber ) || {
+            echo "[ERR] gh release download failed — is the ci-latest release populated?" >&2
             exit 2
+        }
+        ( cd "$dl_dir" && \
+          local stem
+          stem=$(ls *.part-00 2>/dev/null | head -1 | sed 's/\.part-00$//')
+          [ -n "$stem" ] || { echo "[ERR] no .part-00 in download"; exit 2; }
+          log "reassembling $stem from $(ls "$stem".part-* | wc -l) parts"
+          cat "$stem".part-* > "$stem"
+          sha256sum -c *.parts.sha256 || { echo "[ERR] reassembly checksum mismatch"; exit 2; }
+        ) || exit 2
+        ISO=$(ls "$dl_dir"/*.iso 2>/dev/null | head -1)
+        [ -n "$ISO" ] || { echo "[ERR] no iso after reassembly"; exit 2; }
     fi
     if [[ ! -f $ISO ]]; then
         echo "[ERR] ISO not found: $ISO" >&2