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/AppRun

@@ -0,0 +1,5 @@
+#!/bin/sh
+# AppImage entry point: launch the bundled game binary.
+SELF="$(readlink -f "$0")"
+HERE="$(dirname "$SELF")"
+exec "${HERE}/usr/bin/step-line" "$@"

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"

packaging/linux/step-line.appdata.xml

@@ -0,0 +1,21 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<component type="desktop-application">
+  <id>net.step-line.StepLine</id>
+  <name>Step Line</name>
+  <summary>Workplace security simulation on Hollywood Boulevard</summary>
+  <description>
+    <p>
+      Step Line is a workplace security simulation about observation, communication,
+      access verification, and proportionate escalation. Guard the steps and entrance
+      of venues around Hollywood Boulevard: direct tourists, verify wristbands and IDs,
+      screen guests, and escalate only when you have to.
+    </p>
+  </description>
+  <launchable type="desktop-id">step-line.desktop</launchable>
+  <metadata_license>CC0-1.0</metadata_license>
+  <project_license>MIT</project_license>
+  <content_rating type="oars-1.1" />
+  <releases>
+    <release version="0.2.0" date="2026-10-31"/>
+  </releases>
+</component>

packaging/linux/step-line.desktop

@@ -0,0 +1,9 @@
+[Desktop Entry]
+Type=Application
+Name=Step Line
+Comment=Workplace security simulation on Hollywood Boulevard
+Exec=step-line
+Icon=step-line
+Terminal=false
+Categories=Game;Simulation;
+Keywords=security;simulation;hollywood;

packaging/windows/step-line.nsi

@@ -0,0 +1,70 @@
+; Step Line Windows installer (NSIS 3).
+; Build from the repo root with the exported self-contained exe:
+;   makensis /DVERSION=0.2.0 /DARTIFACT=build/step-line-windows-0.2.0.exe \
+;            /DICON=packaging/windows/step-line.ico packaging/windows/step-line.nsi
+; Outputs: build/step-line-setup-<VERSION>.exe
+
+!ifndef VERSION
+  !define VERSION "0.0.0"
+!endif
+!ifndef ARTIFACT
+  !error "ARTIFACT define required (path to exported windows exe)"
+!endif
+!ifndef ICON
+  !define ICON "packaging\windows\step-line.ico"
+!endif
+
+; The installed executable always uses this clean name.
+!define INSTALLED_EXE "Step Line.exe"
+!define COMPANY "Step Line Project"
+!define UNINST_KEY "Software\Microsoft\Windows\CurrentVersion\Uninstall\Step Line"
+
+Unicode true
+ManifestDPIAware true
+Icon "${ICON}"
+UninstallIcon "${ICON}"
+
+Name "Step Line ${VERSION}"
+OutFile "build\step-line-setup-${VERSION}.exe"
+InstallDir "$PROGRAMFILES64\Step Line"
+RequestExecutionLevel admin
+
+Page directory
+Page instfiles
+UninstPage uninstConfirm
+UninstPage instfiles
+
+Section "Install"
+  SetOutPath "$INSTDIR"
+
+  ; The self-contained executable contains ALL game data (embedded PCK).
+  File "/oname=${INSTALLED_EXE}" "${ARTIFACT}"
+
+  WriteUninstaller "$INSTDIR\uninstall.exe"
+
+  ; Shortcuts
+  CreateDirectory "$SMPROGRAMS\Step Line"
+  CreateShortCut "$SMPROGRAMS\Step Line\Step Line.lnk" "$INSTDIR\${INSTALLED_EXE}"
+  CreateShortCut "$SMPROGRAMS\Step Line\Uninstall.lnk" "$INSTDIR\uninstall.exe"
+  CreateShortCut "$DESKTOP\Step Line.lnk" "$INSTDIR\${INSTALLED_EXE}"
+
+  ; Registry (Add/Remove Programs)
+  WriteRegStr HKLM "${UNINST_KEY}" "DisplayName" "Step Line"
+  WriteRegStr HKLM "${UNINST_KEY}" "DisplayVersion" "${VERSION}"
+  WriteRegStr HKLM "${UNINST_KEY}" "Publisher" "${COMPANY}"
+  WriteRegStr HKLM "${UNINST_KEY}" "DisplayIcon" "$INSTDIR\${INSTALLED_EXE}"
+  WriteRegStr HKLM "${UNINST_KEY}" "UninstallString" '"$INSTDIR\uninstall.exe"'
+  WriteRegDWORD HKLM "${UNINST_KEY}" "NoModify" 1
+  WriteRegDWORD HKLM "${UNINST_KEY}" "NoRepair" 1
+SectionEnd
+
+Section "Uninstall"
+  Delete "$INSTDIR\${INSTALLED_EXE}"
+  Delete "$INSTDIR\uninstall.exe"
+  RMDir "$INSTDIR"
+  Delete "$SMPROGRAMS\Step Line\Step Line.lnk"
+  Delete "$SMPROGRAMS\Step Line\Uninstall.lnk"
+  RMDir "$SMPROGRAMS\Step Line"
+  Delete "$DESKTOP\Step Line.lnk"
+  DeleteRegKey HKLM "${UNINST_KEY}"
+SectionEnd