add Shiv-based build for standalone eris executable

- Add 'shiv' to dev dependencies in pyproject.toml
- Add dist/ to .gitignore
- Create packaging/build-shiv script: builds dist/eris zipapp via
  shiv -e eris.__main__:entry_point --compile-pyc --reproducible
- Update uv.lock

packaging/build-shiv

@@ -0,0 +1,26 @@
+#!/bin/bash
+
+
+set -e
+set -x
+
+
+cd "$(dirname "$0")/.."
+mkdir -p dist
+REQS=$(mktemp)
+uv export --no-dev --no-header --no-emit-project --no-hashes > "$REQS"
+uv build --wheel --out-dir dist
+WHEEL=$(ls dist/eris-*.whl 2>/dev/null | head -1)
+if [ -z "$WHEEL" ]; then
+    echo "No wheel found in dist/" >&2
+    exit 1
+fi
+uv run shiv \
+  -e "eris.__main__:entry_point" \
+  -o dist/eris \
+  --compile-pyc \
+  --reproducible \
+  --no-deps \
+  -r "$REQS" \
+  "$WHEEL"
+rm -f "$REQS" "$WHEEL"