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/vision.hpp

@@ -0,0 +1,40 @@
+#pragma once
+
+// Optional OpenCV integration (link alvin::vision).
+
+#include <filesystem>
+#include <optional>
+#include <vector>
+
+#include <opencv2/core.hpp>
+
+#include "alvin/error.hpp"
+#include "alvin/frame.hpp"
+
+namespace alvin::vision {
+
+// Zero-copy CV_8UC4 (BGRA) view. The Frame must outlive the Mat.
+cv::Mat view(const Frame& frame);
+// Deep copy converted to CV_8UC3 (BGR), the layout most OpenCV code expects.
+cv::Mat toBgr(const Frame& frame);
+
+Result<Frame> fromMat(const cv::Mat& mat);
+Result<cv::Mat> loadImage(const std::filesystem::path& path);  // BGR
+Status saveImage(const Frame& frame, const std::filesystem::path& path);  // png/jpg/...
+
+struct Match {
+    Rect rect;          // in frame pixels
+    double score = 0;   // 0..1, higher is better
+};
+
+struct MatchOptions {
+    double threshold = 0.9;
+    bool grayscale = false;  // faster, ignores colour differences
+    int maxResults = 16;     // findAll only
+};
+
+// Normalised cross-correlation template matching.
+Result<std::optional<Match>> find(const Frame& haystack, const cv::Mat& needle, const MatchOptions& opts = {});
+Result<std::vector<Match>> findAll(const Frame& haystack, const cv::Mat& needle, const MatchOptions& opts = {});
+
+}  // namespace alvin::vision