tools/sb3.py --patch removes all assets from .sb3 files

[aspizu]

Repro

tmp=$(mktemp -d /tmp/goboscript-sb3patch-assets-XXXXXX)
printf 'costumes "blank.svg";\n\n' > "$tmp/stage.gs"
printf '<svg xmlns="http://www.w3.org/2000/svg" width="1" height="1"></svg>\n' > "$tmp/blank.svg"
cargo run --quiet -- build "$tmp"
sb3="$tmp/$(basename "$tmp").sb3"
python3 tools/sb3.py "$sb3"
python3 - <<'PY' "$sb3" before
import sys, zipfile
with zipfile.ZipFile(sys.argv[1]) as z:
    print(sys.argv[2], sorted(z.namelist()))
PY
python3 tools/sb3.py --patch "$sb3"
python3 - <<'PY' "$sb3" after
import sys, zipfile
with zipfile.ZipFile(sys.argv[1]) as z:
    print(sys.argv[2], sorted(z.namelist()))
PY

Observed:

before ['19fc47764555c248fbb3cca5465be39b.svg', 'project.json']
after ['project.json']

The docs recommend this workflow for editing generated project.json, but --patch opens the .sb3 in write mode and writes only project.json. Costume and sound assets are dropped, leaving a project that references files no longer present in the archive.

Expected

--patch should replace only project.json and preserve all other zip entries.

[linear]

GS-271