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>

include/alvin/window.hpp

@@ -0,0 +1,64 @@
+#pragma once
+
+#include <cstdint>
+#include <memory>
+#include <optional>
+#include <string>
+#include <string_view>
+#include <vector>
+
+#include "alvin/error.hpp"
+#include "alvin/types.hpp"
+
+namespace alvin {
+
+// Opaque, backend-defined handle: a Hyprland client address, an HWND, ...
+struct WindowId {
+    std::uint64_t value = 0;
+    friend bool operator==(const WindowId&, const WindowId&) = default;
+};
+
+struct WindowInfo {
+    WindowId id;
+    std::string title;
+    std::string appId;  // Wayland app_id / X11 class / Win32 executable name
+    Rect bounds;        // global desktop coordinates
+    std::int64_t pid = 0;
+    bool focused = false;
+    bool visible = true;
+    bool minimized = false;
+    bool fullscreen = false;
+    std::string workspace;  // compositor-specific; empty when not applicable
+};
+
+enum class WindowBackend {
+    Auto,
+    Hyprland,  // Hyprland IPC socket
+    Win32,
+};
+
+std::string_view toString(WindowBackend backend) noexcept;
+
+// Querying and focusing top-level windows. Wayland deliberately hides this
+// from normal clients, so each compositor needs its own implementation.
+class WindowManager {
+public:
+    static Result<std::unique_ptr<WindowManager>> create(WindowBackend backend = WindowBackend::Auto);
+
+    virtual ~WindowManager() = default;
+    WindowManager(const WindowManager&) = delete;
+    WindowManager& operator=(const WindowManager&) = delete;
+
+    virtual WindowBackend backend() const = 0;
+    virtual Result<std::vector<WindowInfo>> list() = 0;
+    virtual Result<std::optional<WindowInfo>> active() = 0;
+    virtual Status focus(WindowId id) = 0;
+
+    // First window whose title or appId contains `needle` (case-insensitive).
+    Result<std::optional<WindowInfo>> find(std::string_view needle);
+
+protected:
+    WindowManager() = default;
+};
+
+}  // namespace alvin