Add Alvin library: input, capture, windows and vision

A cross-platform C++23 library with a platform-neutral public API and
runtime-selected backends:

- Input: Wayland virtual pointer/keyboard (Hyprland, wlroots), libei via
  the RemoteDesktop portal (GNOME, KDE), uinput, and Win32 SendInput
- Capture: ScreenCast portal + PipeWire, and Win32 GDI
- Windows: Hyprland IPC and Win32 enumeration
- Monitors via wl_output/xdg-output and EnumDisplayMonitors
- Optional OpenCV template matching (alvin::vision)

Also adds the `alvin` CLI, Catch2 unit tests, install rules with
find_package(Alvin) support, API docs (docs/API.md and the Obsidian
vault), and README instructions for both ways of consuming the library.

Windows backends are written but not yet compiled.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmake/AlvinConfig.cmake.in

@@ -0,0 +1,18 @@
+@PACKAGE_INIT@
+
+include(CMakeFindDependencyMacro)
+
+# A static libalvin doesn't record its own dependencies, so find them again.
+if(@ALVIN_NEEDS_PKGCONFIG@)
+    find_dependency(PkgConfig)
+@ALVIN_CONFIG_DEPENDENCIES@endif()
+
+set(Alvin_vision_FOUND @ALVIN_HAS_VISION@)
+if(Alvin_vision_FOUND)
+    find_dependency(OpenCV COMPONENTS core imgproc imgcodecs)
+endif()
+
+include("${CMAKE_CURRENT_LIST_DIR}/AlvinTargets.cmake")
+
+# find_package(Alvin REQUIRED COMPONENTS vision) fails if Alvin was built without OpenCV.
+check_required_components(Alvin)