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/error.hpp

@@ -0,0 +1,38 @@
+#pragma once
+
+#include <expected>
+#include <string>
+#include <string_view>
+
+namespace alvin {
+
+enum class ErrorCode {
+    Unknown,
+    Unsupported,       // backend or feature not available on this platform/session
+    NotAvailable,      // backend compiled in but its service/device isn't present
+    PermissionDenied,  // e.g. /dev/uinput not writable, portal request denied
+    Cancelled,         // user dismissed a portal dialog
+    Timeout,
+    InvalidArgument,
+    NotFound,
+    Io,
+    Protocol,          // unexpected reply from a compositor / portal / IPC socket
+};
+
+struct Error {
+    ErrorCode code = ErrorCode::Unknown;
+    std::string message;
+};
+
+template <class T>
+using Result = std::expected<T, Error>;
+
+using Status = Result<void>;
+
+inline std::unexpected<Error> fail(ErrorCode code, std::string message) {
+    return std::unexpected(Error{code, std::move(message)});
+}
+
+std::string_view toString(ErrorCode code) noexcept;
+
+}  // namespace alvin