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, 91 insertions, 0 deletions

diff --git a/legacy-cpp/src/platform/platform.h b/legacy-cpp/src/platform/platform.h
new file mode 100644
index 0000000..41cc6bf
--- /dev/null
+++ b/legacy-cpp/src/platform/platform.h
@@ -0,0 +1,91 @@
+#ifndef SRDWM_PLATFORM_H
+#define SRDWM_PLATFORM_H
+
+#include <memory>
+#include <vector>
+#include <string>
+#include <functional>
+
+// Forward declarations
+class SRDWindow;
+
+// Include Monitor struct definition from layouts
+#include "../layouts/layout.h"
+
+// Platform-independent event types
+enum class EventType {
+    WindowCreated,
+    WindowDestroyed,
+    WindowMoved,
+    WindowResized,
+    WindowFocused,
+    WindowUnfocused,
+    KeyPress,
+    KeyRelease,
+    MouseButtonPress,
+    MouseButtonRelease,
+    MouseMotion,
+    MonitorAdded,
+    MonitorRemoved
+};
+
+// Event structure
+struct Event {
+    EventType type;
+    void* data;
+    size_t data_size;
+};
+
+// Platform abstraction interface
+class Platform {
+public:
+    virtual ~Platform() = default;
+    
+    // Initialization and cleanup
+    virtual bool initialize() = 0;
+    virtual void shutdown() = 0;
+    
+    // Event handling
+    virtual bool poll_events(std::vector<Event>& events) = 0;
+    virtual void process_event(const Event& event) = 0;
+    
+    // Window management
+    virtual std::unique_ptr<SRDWindow> create_window(const std::string& title, int x, int y, int width, int height) = 0;
+    virtual void destroy_window(SRDWindow* window) = 0;
+    virtual void set_window_position(SRDWindow* window, int x, int y) = 0;
+    virtual void set_window_size(SRDWindow* window, int width, int height) = 0;
+    virtual void set_window_title(SRDWindow* window, const std::string& title) = 0;
+    virtual void focus_window(SRDWindow* window) = 0;
+    virtual void minimize_window(SRDWindow* window) = 0;
+    virtual void maximize_window(SRDWindow* window) = 0;
+    virtual void close_window(SRDWindow* window) = 0;
+    
+    // Window decorations (cross-platform)
+    virtual void set_window_decorations(SRDWindow* window, bool enabled) = 0;
+    virtual void set_window_border_color(SRDWindow* window, int r, int g, int b) = 0;
+    virtual void set_window_border_width(SRDWindow* window, int width) = 0;
+    virtual bool get_window_decorations(SRDWindow* window) const = 0;
+    
+    // Monitor management
+    virtual std::vector<Monitor> get_monitors() = 0;
+    virtual Monitor get_primary_monitor() = 0;
+    
+    // Input handling
+    virtual void grab_keyboard() = 0;
+    virtual void ungrab_keyboard() = 0;
+    virtual void grab_pointer() = 0;
+    virtual void ungrab_pointer() = 0;
+    
+    // Utility
+    virtual std::string get_platform_name() const = 0;
+    virtual bool is_wayland() const = 0;
+    virtual bool is_x11() const = 0;
+    virtual bool is_windows() const = 0;
+    virtual bool is_macos() const = 0;
+};
+
+// Forward declaration
+class PlatformFactory;
+
+#endif // SRDWM_PLATFORM_H
+