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/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