Release workflow: Linux tar.gz and AppImage, Windows zip, universal macOS app, web zip, SHA256SUMS

.github/workflows/release.yml builds on a pushed v* tag (or by hand) and attaches everything to a
GitHub release, a pre-release while the version is 0.x, with .github/release-notes.md. The
packaging lives in .github/release/package-{linux,macos}.sh so it can be run locally; the
AppImage icon is rendered by solvecraft-cli from the bracket example.

Fixes #32

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/release/package-linux.sh

@@ -0,0 +1,39 @@
+#!/usr/bin/env bash
+# Package the Linux release build: a tar.gz with both binaries, and an AppImage of the desktop app.
+# Usage: .github/release/package-linux.sh <tag>   (from the repository root, after
+# `cargo build --release -p solvecraft -p solvecraft-cli`). Writes out/.
+set -euo pipefail
+tag=${1:?usage: package-linux.sh <tag>}
+bin=${CARGO_TARGET_DIR:-target}/release
+name="solvecraft-$tag-linux-x86_64"
+rm -rf stage out && mkdir -p "stage/$name" out
+cp "$bin/solvecraft" "$bin/solvecraft-cli" README.md LICENSE-MIT LICENSE-APACHE NOTICE ATTRIBUTION.md "stage/$name/"
+tar -C stage -czf "out/$name.tar.gz" "$name"
+
+# AppImage. The icon is rendered by SolveCraft itself from the bracket example, so no image file
+# is committed.
+app=stage/SolveCraft.AppDir
+mkdir -p "$app/usr/bin"
+cp "$bin/solvecraft" "$bin/solvecraft-cli" "$app/usr/bin/"
+"$bin/solvecraft-cli" snapshot examples/bracket.json --out "$app/solvecraft.png" --width 256 --height 256 >/dev/null
+cat > "$app/solvecraft.desktop" <<'DESKTOP'
+[Desktop Entry]
+Type=Application
+Name=SolveCraft
+Comment=Parametric 3D CAD
+Exec=solvecraft %f
+Icon=solvecraft
+Categories=Graphics;Engineering;
+MimeType=model/step;
+DESKTOP
+cat > "$app/AppRun" <<'APPRUN'
+#!/bin/sh
+here="$(dirname "$(readlink -f "$0")")"
+exec "$here/usr/bin/solvecraft" "$@"
+APPRUN
+chmod +x "$app/AppRun"
+tool=stage/appimagetool
+curl -sSfL -o "$tool" https://github.com/AppImage/appimagetool/releases/download/continuous/appimagetool-x86_64.AppImage
+chmod +x "$tool"
+APPIMAGE_EXTRACT_AND_RUN=1 ARCH=x86_64 "$tool" --no-appstream "$app" "out/SolveCraft-$tag-x86_64.AppImage"
+ls -l out