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, 70 insertions, 0 deletions

diff --git a/legacy-cpp/src/layouts/layout_engine.h b/legacy-cpp/src/layouts/layout_engine.h
new file mode 100644
index 0000000..d23033c
--- /dev/null
+++ b/legacy-cpp/src/layouts/layout_engine.h
@@ -0,0 +1,70 @@
+#ifndef SRDWM_LAYOUT_ENGINE_H
+#define SRDWM_LAYOUT_ENGINE_H
+
+#include "layout.h"
+#include "tiling_layout.h"
+#include "dynamic_layout.h"
+#include <vector>
+#include <map>
+#include <string>
+#include <functional>
+
+class SRDWindow; // Forward declaration to avoid circular dependency
+
+enum class LayoutType {
+    TILING,
+    DYNAMIC,
+    FLOATING
+    // Add other layout types here later
+};
+
+class LayoutEngine {
+public:
+    LayoutEngine();
+    ~LayoutEngine();
+
+    // Layout management
+    bool set_layout(int monitor_id, LayoutType layout_type);
+    bool set_layout(int monitor_id, const std::string& layout_name);
+    LayoutType get_layout(int monitor_id) const;
+    std::string get_layout_name(int monitor_id) const;
+    
+    // Layout configuration
+    bool configure_layout(const std::string& layout_name, const std::map<std::string, std::string>& config);
+    bool register_custom_layout(const std::string& name, std::function<void(const std::vector<SRDWindow*>&, const Monitor&)> layout_func);
+    
+    // SRDWindow management
+    void add_window(SRDWindow* window);
+    void remove_window(SRDWindow* window);
+    void update_window(SRDWindow* window);
+    
+    // Monitor management
+    void add_monitor(const Monitor& monitor);
+    void remove_monitor(int monitor_id);
+    void update_monitor(const Monitor& monitor);
+    
+    // Arrangement
+    void arrange_on_monitor(const Monitor& monitor);
+    void arrange_all_monitors();
+    
+    // Utility
+    std::vector<std::string> get_available_layouts() const;
+    std::vector<SRDWindow*> get_windows_on_monitor(int monitor_id) const;
+
+private:
+    // Member variables for layout state
+    std::vector<Monitor> monitors_;
+    std::vector<SRDWindow*> windows_;
+    TilingLayout tiling_layout_;
+    DynamicLayout dynamic_layout_;
+    std::map<int, LayoutType> active_layouts_; // Map monitor ID to active layout type
+    std::map<std::string, std::function<void(const std::vector<SRDWindow*>&, const Monitor&)>> custom_layouts_;
+    std::map<std::string, std::map<std::string, std::string>> layout_configs_;
+    
+    // Helper methods
+    LayoutType string_to_layout_type(const std::string& name) const;
+    std::string layout_type_to_string(LayoutType type) const;
+    bool is_window_on_monitor(const SRDWindow* window, const Monitor& monitor) const;
+};
+
+#endif // SRDWM_LAYOUT_ENGINE_H