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, 221 insertions, 0 deletions

diff --git a/crates/core/src/window.rs b/crates/core/src/window.rs
new file mode 100644
index 0000000..3970b02
--- /dev/null
+++ b/crates/core/src/window.rs
@@ -0,0 +1,221 @@
+use crate::geometry::Rect;
+
+pub type WindowId = u64;
+
+/// State of a single managed window. This is platform-independent: backends
+/// (X11, Wayland, ...) own the real surface/client handle and keep a `Window`
+/// in sync with it via `srdwm_core::WindowManager`.
+#[derive(Debug, Clone)]
+pub struct Window {
+    pub id: WindowId,
+    pub title: String,
+    pub app_id: String,
+    pub geometry: Rect,
+    /// Geometry to restore to when un-maximizing.
+    pub restore_geometry: Option<Rect>,
+    pub decorated: bool,
+    pub floating: bool,
+    pub minimized: bool,
+    pub maximized: bool,
+    pub fullscreen: bool,
+    pub always_on_top: bool,
+    pub border_color: (u8, u8, u8),
+    pub border_width: u32,
+    pub workspace: usize,
+    pub monitor: u32,
+}
+
+impl Window {
+    pub fn new(id: WindowId, title: impl Into<String>) -> Self {
+        Self {
+            id,
+            title: title.into(),
+            app_id: String::new(),
+            geometry: Rect::new(0, 0, 640, 480),
+            restore_geometry: None,
+            decorated: true,
+            floating: false,
+            minimized: false,
+            maximized: false,
+            fullscreen: false,
+            always_on_top: false,
+            border_color: (136, 192, 208), // Nord accent, matches legacy theme default
+            border_width: 2,
+            workspace: 0,
+            monitor: 0,
+        }
+    }
+}
+
+/// The height, in pixels, of the drawn title bar. Shared between backends so
+/// hit-testing and rendering agree on the same band.
+pub const TITLEBAR_HEIGHT: u32 = 30;
+/// Width of a resize grab margin along each window edge.
+pub const RESIZE_MARGIN: i32 = 6;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum ResizeEdge {
+    Left,
+    Right,
+    Top,
+    Bottom,
+    TopLeft,
+    TopRight,
+    BottomLeft,
+    BottomRight,
+}
+
+impl ResizeEdge {
+    /// Determine which titlebar button (if any) a point within the titlebar
+    /// band falls on. Buttons are laid out right-aligned: close, maximize, minimize.
+    pub fn hit_test(frame: Rect, x: i32, y: i32) -> Option<TitlebarHit> {
+        if !frame.contains_point(x, y) {
+            return None;
+        }
+        if y < frame.y + TITLEBAR_HEIGHT as i32 {
+            const BUTTON: i32 = TITLEBAR_HEIGHT as i32;
+            let right = frame.right();
+            if x >= right - BUTTON {
+                return Some(TitlebarHit::Close);
+            }
+            if x >= right - BUTTON * 2 {
+                return Some(TitlebarHit::Maximize);
+            }
+            if x >= right - BUTTON * 3 {
+                return Some(TitlebarHit::Minimize);
+            }
+            return Some(TitlebarHit::Drag);
+        }
+        let edge = Self::resize_edge_at(frame, x, y)?;
+        Some(TitlebarHit::Resize(edge))
+    }
+
+    fn resize_edge_at(frame: Rect, x: i32, y: i32) -> Option<ResizeEdge> {
+        let m = RESIZE_MARGIN;
+        let near_left = x <= frame.x + m;
+        let near_right = x >= frame.right() - m;
+        let near_top = y <= frame.y + m;
+        let near_bottom = y >= frame.bottom() - m;
+        Some(match (near_left, near_right, near_top, near_bottom) {
+            (true, _, true, _) => ResizeEdge::TopLeft,
+            (_, true, true, _) => ResizeEdge::TopRight,
+            (true, _, _, true) => ResizeEdge::BottomLeft,
+            (_, true, _, true) => ResizeEdge::BottomRight,
+            (true, false, false, false) => ResizeEdge::Left,
+            (false, true, false, false) => ResizeEdge::Right,
+            (false, false, false, true) => ResizeEdge::Bottom,
+            _ => return None,
+        })
+    }
+
+    /// Apply a pointer delta to `original` geometry along this edge, honoring
+    /// the given minimum size.
+    pub fn apply_delta(self, original: Rect, dx: i32, dy: i32, min_w: u32, min_h: u32) -> Rect {
+        let mut r = original;
+        let min_w = min_w as i32;
+        let min_h = min_h as i32;
+        match self {
+            ResizeEdge::Left | ResizeEdge::TopLeft | ResizeEdge::BottomLeft => {
+                let new_w = (original.width as i32 - dx).max(min_w);
+                r.x = original.right() - new_w;
+                r.width = new_w as u32;
+            }
+            _ => {}
+        }
+        match self {
+            ResizeEdge::Right | ResizeEdge::TopRight | ResizeEdge::BottomRight => {
+                r.width = (original.width as i32 + dx).max(min_w) as u32;
+            }
+            _ => {}
+        }
+        match self {
+            ResizeEdge::Top | ResizeEdge::TopLeft | ResizeEdge::TopRight => {
+                let new_h = (original.height as i32 - dy).max(min_h);
+                r.y = original.bottom() - new_h;
+                r.height = new_h as u32;
+            }
+            _ => {}
+        }
+        match self {
+            ResizeEdge::Bottom | ResizeEdge::BottomLeft | ResizeEdge::BottomRight => {
+                r.height = (original.height as i32 + dy).max(min_h) as u32;
+            }
+            _ => {}
+        }
+        r
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum TitlebarHit {
+    Drag,
+    Close,
+    Maximize,
+    Minimize,
+    Resize(ResizeEdge),
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn frame() -> Rect {
+        Rect::new(100, 100, 400, 300)
+    }
+
+    #[test]
+    fn close_button_is_top_right_corner_of_titlebar() {
+        let f = frame();
+        let hit = ResizeEdge::hit_test(f, f.right() - 5, f.y + 5);
+        assert_eq!(hit, Some(TitlebarHit::Close));
+    }
+
+    #[test]
+    fn maximize_is_left_of_close() {
+        let f = frame();
+        let hit = ResizeEdge::hit_test(f, f.right() - TITLEBAR_HEIGHT as i32 - 5, f.y + 5);
+        assert_eq!(hit, Some(TitlebarHit::Maximize));
+    }
+
+    #[test]
+    fn middle_of_titlebar_is_drag() {
+        let f = frame();
+        let (cx, _) = f.center();
+        let hit = ResizeEdge::hit_test(f, cx, f.y + 5);
+        assert_eq!(hit, Some(TitlebarHit::Drag));
+    }
+
+    #[test]
+    fn bottom_right_corner_is_resize() {
+        let f = frame();
+        let hit = ResizeEdge::hit_test(f, f.right() - 1, f.bottom() - 1);
+        assert_eq!(hit, Some(TitlebarHit::Resize(ResizeEdge::BottomRight)));
+    }
+
+    #[test]
+    fn outside_frame_is_none() {
+        let f = frame();
+        assert_eq!(ResizeEdge::hit_test(f, 0, 0), None);
+    }
+
+    #[test]
+    fn resize_right_edge_grows_width_only() {
+        let r = Rect::new(0, 0, 200, 100);
+        let out = ResizeEdge::Right.apply_delta(r, 50, 999, 50, 50);
+        assert_eq!(out, Rect::new(0, 0, 250, 100));
+    }
+
+    #[test]
+    fn resize_left_edge_moves_x_and_shrinks_width() {
+        let r = Rect::new(100, 0, 200, 100);
+        let out = ResizeEdge::Left.apply_delta(r, 30, 0, 50, 50);
+        assert_eq!(out, Rect::new(130, 0, 170, 100));
+    }
+
+    #[test]
+    fn resize_respects_minimum_size() {
+        let r = Rect::new(0, 0, 100, 100);
+        let out = ResizeEdge::Right.apply_delta(r, -500, 0, 50, 50);
+        assert_eq!(out.width, 50);
+    }
+}