Rewrite srdwm in Rust: working X11 and Wayland backends, Lua config

The C++ prototype (moved to legacy-cpp/) was mostly a design skeleton:
X11 and Windows backends were partially real, Wayland created the
wlroots object graph but never wired a single event listener, macOS
was stub except monitor enumeration, and the Lua engine's srd.bind()
stored a key-combo string but never the actual closure. See
docs/PRIOR_ART.md for the full audit.

This replaces it with a Cargo workspace:

- srdwm-core: platform-independent window/workspace/monitor state,
  a real master-stack tiling layout, and SmartPlacement grid/cascade/
  snap-to-edge placement - fixing several bugs in the C++ version
  (hardcoded 2-column grid, cascade that never cascaded, snap-to-edge
  that always returned a fixed rect). 35 unit tests.
- srdwm-config: the srd Lua API via mlua, implementing the surface
  docs/DEFAULTS.md always documented but the C++ engine never actually
  built (srd.window.close()/focus(direction), srd.workspace.next(),
  real keybinding closures, require("srd") support). 10 unit tests.
- srdwm-x11: a real reparenting WM with a drawn title bar (buttons,
  drag, resize), verified live under Xephyr - frame placement and
  client offset match srdwm-core's computed geometry exactly, and the
  decoration renders correctly on screen.
- srdwm-wayland: a from-scratch smithay compositor (the C++ version
  had nothing working to port from) - runs via the winit backend,
  tracks xdg-shell toplevels through the same WindowManager and
  hit-testing code X11 uses, verified to start/render/run without
  crashing. Decorations are solid-color (no text yet); see
  docs/IMPLEMENTATION_STATUS.md for exact scope.
- srdwm-windows / srdwm-macos: structured, cfg-gated designs informed
  by komorebi/glazewm and yabai/AeroSpace respectively (see
  docs/PRIOR_ART.md), honestly marked as unbuilt/unverified since this
  sandbox has no Windows or macOS target.

1 files changed, 105 insertions, 0 deletions

diff --git a/legacy-cpp/src/platform/linux_platform.h b/legacy-cpp/src/platform/linux_platform.h
new file mode 100644
index 0000000..1237c85
--- /dev/null
+++ b/legacy-cpp/src/platform/linux_platform.h
@@ -0,0 +1,105 @@
+#ifndef SRDWM_LINUX_PLATFORM_H
+#define SRDWM_LINUX_PLATFORM_H
+
+#include "platform.h"
+#include <memory>
+#include <string>
+
+// Forward declarations for X11
+#ifdef __linux__
+struct _XDisplay;
+typedef struct _XDisplay Display;
+typedef unsigned long SRDWindow;
+typedef unsigned long Atom;
+
+// Forward declarations for Wayland
+struct wl_display;
+struct wl_registry;
+struct wl_compositor;
+struct wl_shell;
+struct wl_seat;
+struct wl_keyboard;
+struct wl_pointer;
+#endif
+
+class LinuxPlatform : public Platform {
+public:
+    enum class Backend {
+        Auto,
+        X11,
+        Wayland
+    };
+
+    explicit LinuxPlatform(Backend backend = Backend::Auto);
+    ~LinuxPlatform() override;
+
+    // Platform interface implementation
+    bool initialize() override;
+    void shutdown() override;
+    
+    bool poll_events(std::vector<Event>& events) override;
+    void process_event(const Event& event) override;
+    
+    std::unique_ptr<SRDWindow> create_window(const std::string& title, int x, int y, int width, int height) override;
+    void destroy_window(SRDWindow* window) override;
+    void set_window_position(SRDWindow* window, int x, int y) override;
+    void set_window_size(SRDWindow* window, int width, int height) override;
+    void set_window_title(SRDWindow* window, const std::string& title) override;
+    void focus_window(SRDWindow* window) override;
+    void minimize_window(SRDWindow* window) override;
+    void maximize_window(SRDWindow* window) override;
+    void close_window(SRDWindow* window) override;
+    
+    std::vector<Monitor> get_monitors() override;
+    Monitor get_primary_monitor() override;
+    
+    void grab_keyboard() override;
+    void ungrab_keyboard() override;
+    void grab_pointer() override;
+    void ungrab_pointer() override;
+    
+    std::string get_platform_name() const override;
+    bool is_wayland() const override;
+    bool is_x11() const override;
+    bool is_windows() const override;
+    bool is_macos() const override;
+
+private:
+    Backend backend_;
+    bool initialized_;
+    
+    // X11 specific members
+    Display* x11_display_;
+    SRDWindow x11_root_;
+    Atom x11_wm_delete_window_;
+    Atom x11_wm_protocols_;
+    
+    // Wayland specific members
+    wl_display* wayland_display_;
+    wl_registry* wayland_registry_;
+    wl_compositor* wayland_compositor_;
+    wl_shell* wayland_shell_;
+    wl_seat* wayland_seat_;
+    wl_keyboard* wayland_keyboard_;
+    wl_pointer* wayland_pointer_;
+    
+    // Common members
+    std::vector<Monitor> monitors_;
+    
+    // Private methods
+    bool initialize_x11();
+    bool initialize_wayland();
+    void shutdown_x11();
+    void shutdown_wayland();
+    
+    bool detect_backend();
+    void setup_x11_atoms();
+    void setup_wayland_registry();
+    
+    // Event processing
+    void process_x11_event(XEvent& event, std::vector<Event>& events);
+    void process_wayland_event(wl_display* display, std::vector<Event>& events);
+};
+
+#endif // SRDWM_LINUX_PLATFORM_H
+