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/types.hpp

@@ -0,0 +1,58 @@
+#pragma once
+
+#include <cstdint>
+#include <string>
+#include <vector>
+
+#include "alvin/error.hpp"
+
+namespace alvin {
+
+struct Point {
+    int x = 0;
+    int y = 0;
+    friend bool operator==(const Point&, const Point&) = default;
+};
+
+struct Size {
+    int width = 0;
+    int height = 0;
+    friend bool operator==(const Size&, const Size&) = default;
+};
+
+// All rectangles are in global desktop coordinates (logical pixels, i.e. after
+// monitor scaling) unless documented otherwise.
+struct Rect {
+    int x = 0;
+    int y = 0;
+    int width = 0;
+    int height = 0;
+
+    int right() const { return x + width; }
+    int bottom() const { return y + height; }
+    Point center() const { return {x + width / 2, y + height / 2}; }
+    bool empty() const { return width <= 0 || height <= 0; }
+    bool contains(Point p) const { return p.x >= x && p.y >= y && p.x < right() && p.y < bottom(); }
+
+    // Smallest rect containing both. An empty rect acts as identity.
+    Rect united(const Rect& o) const;
+
+    friend bool operator==(const Rect&, const Rect&) = default;
+};
+
+struct Monitor {
+    std::string name;       // connector name, e.g. "DP-1" or "\\\\.\\DISPLAY1"
+    std::string description;
+    Rect bounds;            // logical position/size in the global desktop
+    Size pixelSize;         // physical mode size in pixels
+    double scale = 1.0;
+    bool primary = false;
+};
+
+// Enumerate monitors on the current session.
+Result<std::vector<Monitor>> listMonitors();
+
+// Bounding box of all monitors, i.e. the space absolute pointer motion maps onto.
+Result<Rect> desktopBounds();
+
+}  // namespace alvin