Add Windows installer (NSIS) and Linux AppImage packaging; embed PCK in all exports

- export_presets: embed_pck=true for Linux/Windows/macOS -> single self-contained
  binaries (all game data in one file); build_releases.sh cleans stale .pck
- App icon: assets/textures/icon.png (1024) + multi-size icon.ico generated by
  tools/generate_textures.py; copied into packaging/windows and packaging/linux
- packaging/windows/step-line.nsi: NSIS installer (shortcuts, uninstaller,
  registry Add/Remove Programs) output step-line-setup-<ver>.exe
- packaging/linux: step-line.desktop, AppRun, step-line.appdata.xml,
  build_appimage.sh (appimagetool, APPIMAGE_EXTRACT_AND_RUN) ->
  step-line-<ver>-x86_64.AppImage
- CI build job now packages NSIS installer + AppImage alongside binaries;
  publish glob already uploads them
- Local end-to-end verification: embedded exports rebuilt, AppImage built and
  run headless (0 script errors); suite 57/57
- README Downloads table lists installer/AppImage/portable rows; RELEASING
  documents packaging; TODO/DONE synced

packaging/linux/build_appimage.sh

@@ -0,0 +1,45 @@
+#!/usr/bin/env bash
+# Build a self-contained Step Line AppImage (all game data included).
+#
+# Prereqs: exported Linux binary in build/ (embed_pck=true), curl.
+# Usage:   bash packaging/linux/build_appimage.sh [version]
+# Output:  build/step-line-<version>-x86_64.AppImage
+set -euo pipefail
+
+HERE="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+ROOT="$(cd "$HERE/../.." && pwd)"
+VERSION="${1:-$(sed -n 's/.*config\/version="\([^"]*\)".*/\1/p' "$ROOT/project.godot")}"
+SRC_BIN="$ROOT/build/step-line-linux-${VERSION}.x86_64"
+
+if [[ ! -f "$SRC_BIN" ]]; then
+  echo "error: missing $SRC_BIN — run ./tools/build_releases.sh first" >&2
+  exit 1
+fi
+
+WORK="$(mktemp -d)"
+trap 'rm -rf "$WORK"' EXIT
+APPDIR="$WORK/StepLine.AppDir"
+mkdir -p "$APPDIR/usr/bin"
+cp "$SRC_BIN" "$APPDIR/usr/bin/step-line"
+# The export embeds the PCK (embed_pck=true), so no extra data files are needed.
+
+cp "$HERE/step-line.desktop" "$APPDIR/step-line.desktop"
+cp "$HERE/AppRun" "$APPDIR/AppRun"
+chmod +x "$APPDIR/AppRun"
+cp "$HERE/step-line.png" "$APPDIR/step-line.png"
+mkdir -p "$APPDIR/usr/share/metainfo"
+cp "$HERE/step-line.appdata.xml" "$APPDIR/usr/share/metainfo/step-line.appdata.xml"
+
+TOOL="${APPIMAGE_TOOL:-/tmp/appimagetool-x86_64.AppImage}"
+if [[ ! -f "$TOOL" ]]; then
+  echo "==> Downloading appimagetool"
+  curl -sL -o "$TOOL" \
+    https://github.com/AppImage/appimagetool/releases/download/continuous/appimagetool-x86_64.AppImage
+  chmod +x "$TOOL"
+fi
+
+OUT="$ROOT/build/step-line-${VERSION}-x86_64.AppImage"
+echo "==> Building AppImage -> $OUT"
+# Extract-and-run avoids a FUSE dependency on the build host.
+APPIMAGE_EXTRACT_AND_RUN=1 "$TOOL" "$APPDIR" "$OUT"
+echo "AppImage ready: $(ls -la "$OUT" | awk '{print $5}') bytes"