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, 62 insertions, 0 deletions

diff --git a/legacy-cpp/src/layouts/smart_placement.h b/legacy-cpp/src/layouts/smart_placement.h
new file mode 100644
index 0000000..69f4b94
--- /dev/null
+++ b/legacy-cpp/src/layouts/smart_placement.h
@@ -0,0 +1,62 @@
+#ifndef SRDWM_SMART_PLACEMENT_H
+#define SRDWM_SMART_PLACEMENT_H
+
+#include "layout.h"
+#include <vector>
+#include <memory>
+
+// Forward declarations
+class SRDWindow;
+class Monitor;
+
+// Smart placement algorithm that mimics SRDWindows 11 behavior
+class SmartPlacement {
+public:
+    struct PlacementResult {
+        int x, y, width, height;
+        bool success;
+        std::string reason;
+    };
+
+    // Main placement function
+    static PlacementResult place_window(const SRDWindow* window, const Monitor& monitor, 
+                                       const std::vector<SRDWindow*>& existing_windows);
+
+    // Grid-based placement (SRDWindows 11 style)
+    static PlacementResult place_in_grid(const SRDWindow* window, const Monitor& monitor,
+                                        const std::vector<SRDWindow*>& existing_windows);
+
+    // Snap-to-edge placement
+    static PlacementResult snap_to_edge(const SRDWindow* window, const Monitor& monitor,
+                                       const std::vector<SRDWindow*>& existing_windows);
+
+    // Cascade placement for overlapping windows
+    static PlacementResult cascade_place(const SRDWindow* window, const Monitor& monitor,
+                                        const std::vector<SRDWindow*>& existing_windows);
+
+    // Smart tiling placement
+    static PlacementResult smart_tile(const SRDWindow* window, const Monitor& monitor,
+                                     const std::vector<SRDWindow*>& existing_windows);
+
+private:
+    // Helper functions
+    static bool windows_overlap(const SRDWindow* w1, const SRDWindow* w2);
+    static int calculate_overlap_score(const SRDWindow* window, const Monitor& monitor,
+                                      const std::vector<SRDWindow*>& existing_windows);
+    static std::vector<std::pair<int, int>> find_free_spaces(const Monitor& monitor,
+                                                             const std::vector<SRDWindow*>& existing_windows);
+    static bool is_position_valid(int x, int y, int width, int height, const Monitor& monitor);
+    
+    // Grid calculations
+    static std::pair<int, int> calculate_grid_position(const SRDWindow* window, const Monitor& monitor);
+    static int calculate_optimal_grid_size(const Monitor& monitor, int window_count);
+    
+    // Constants
+    static constexpr int MIN_WINDOW_WIDTH = 200;
+    static constexpr int MIN_WINDOW_HEIGHT = 150;
+    static constexpr int GRID_MARGIN = 10;
+    static constexpr int CASCADE_OFFSET = 30;
+};
+
+#endif // SRDWM_SMART_PLACEMENT_H
+