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/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>