feat(windows): icon, windowed exe, no-admin installer

- PyInstaller exe built windowed (console=False) with package/testium.ico
  as the embedded icon (BMP entries for shell compatibility).
- Suppress stray subprocess console windows in the frozen Windows build via
  paths.no_window_kwargs() (CREATE_NO_WINDOW); wheel/source unchanged.
  Applied to py_process, lua_process, bins probes, sys_app_path_win.
- New per-user Inno Setup installer (package/innosetup/): no admin,
  version-scoped AppId/dir so versions install side-by-side, one Start
  Menu entry per version, .ico shipped for shortcut/uninstall icons.
- DESIGN.md + release_note.txt updated.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

package/pyinstaller/testium.spec

@@ -94,11 +94,11 @@ exe = EXE(
     upx=not os.environ.get("TESTIUM_NO_UPX"),
     upx_exclude=[],
     runtime_tmpdir=None,
-    console=True,
+    console=False,
     disable_windowed_traceback=False,
     argv_emulation=False,
     target_arch=None,
     codesign_identity=None,
     entitlements_file=None,
-    ico='../testium.png'
+    ico='../testium.ico'
 )