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>

tests/CMakeLists.txt

@@ -0,0 +1,22 @@
+find_package(Catch2 3 QUIET)
+if(NOT Catch2_FOUND)
+    include(FetchContent)
+    FetchContent_Declare(Catch2
+        GIT_REPOSITORY https://github.com/catchorg/Catch2.git
+        GIT_TAG v3.7.1)
+    FetchContent_MakeAvailable(Catch2)
+endif()
+
+add_executable(alvin_tests
+    test_core.cpp
+    test_json.cpp)
+if(CMAKE_SYSTEM_NAME STREQUAL "Linux")
+    target_sources(alvin_tests PRIVATE test_linux.cpp)
+    target_link_libraries(alvin_tests PRIVATE PkgConfig::XKBCOMMON)
+endif()
+# Tests exercise internal helpers too, so they see the private source tree.
+target_include_directories(alvin_tests PRIVATE ${PROJECT_SOURCE_DIR}/src)
+target_link_libraries(alvin_tests PRIVATE alvin::alvin Catch2::Catch2WithMain)
+
+include(Catch)
+catch_discover_tests(alvin_tests)