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, 77 insertions, 0 deletions

diff --git a/crates/core/src/geometry.rs b/crates/core/src/geometry.rs
new file mode 100644
index 0000000..75feeef
--- /dev/null
+++ b/crates/core/src/geometry.rs
@@ -0,0 +1,77 @@
+/// An axis-aligned rectangle in screen space, used for window and monitor bounds.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub struct Rect {
+    pub x: i32,
+    pub y: i32,
+    pub width: u32,
+    pub height: u32,
+}
+
+impl Rect {
+    pub fn new(x: i32, y: i32, width: u32, height: u32) -> Self {
+        Self { x, y, width, height }
+    }
+
+    pub fn right(&self) -> i32 {
+        self.x + self.width as i32
+    }
+
+    pub fn bottom(&self) -> i32 {
+        self.y + self.height as i32
+    }
+
+    pub fn contains_point(&self, x: i32, y: i32) -> bool {
+        x >= self.x && x < self.right() && y >= self.y && y < self.bottom()
+    }
+
+    pub fn overlaps(&self, other: &Rect) -> bool {
+        !(self.right() <= other.x
+            || other.right() <= self.x
+            || self.bottom() <= other.y
+            || other.bottom() <= self.y)
+    }
+
+    /// Shrinks the rect on all sides by `margin`, saturating at zero size.
+    pub fn inset(&self, margin: u32) -> Rect {
+        let m = margin as i32;
+        let width = self.width.saturating_sub(margin * 2);
+        let height = self.height.saturating_sub(margin * 2);
+        Rect { x: self.x + m, y: self.y + m, width, height }
+    }
+
+    pub fn center(&self) -> (i32, i32) {
+        (self.x + self.width as i32 / 2, self.y + self.height as i32 / 2)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn overlap_detection_matches_aabb_semantics() {
+        let a = Rect::new(0, 0, 100, 100);
+        let b = Rect::new(50, 50, 100, 100);
+        let c = Rect::new(100, 100, 50, 50); // touches corner, should not overlap (half-open)
+        let d = Rect::new(200, 200, 10, 10);
+
+        assert!(a.overlaps(&b));
+        assert!(!a.overlaps(&c));
+        assert!(!a.overlaps(&d));
+    }
+
+    #[test]
+    fn inset_shrinks_symmetrically() {
+        let r = Rect::new(0, 0, 100, 60);
+        let inset = r.inset(10);
+        assert_eq!(inset, Rect::new(10, 10, 80, 40));
+    }
+
+    #[test]
+    fn contains_point_is_half_open() {
+        let r = Rect::new(0, 0, 10, 10);
+        assert!(r.contains_point(0, 0));
+        assert!(!r.contains_point(10, 10));
+        assert!(r.contains_point(9, 9));
+    }
+}