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.

2 files changed, 110 insertions, 0 deletions

diff --git a/crates/platform/Cargo.toml b/crates/platform/Cargo.toml
new file mode 100644
index 0000000..d94723e
--- /dev/null
+++ b/crates/platform/Cargo.toml
@@ -0,0 +1,11 @@
+[package]
+name = "srdwm-platform"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+description = "Platform abstraction trait shared by all srdwm backends"
+
+[dependencies]
+srdwm-core.workspace = true
+log.workspace = true
+thiserror.workspace = true
diff --git a/crates/platform/src/lib.rs b/crates/platform/src/lib.rs
new file mode 100644
index 0000000..fdad291
--- /dev/null
+++ b/crates/platform/src/lib.rs
@@ -0,0 +1,99 @@
+//! Platform abstraction: the trait every backend (X11, Wayland, Windows,
+//! macOS) implements, plus auto-detection of which backend to use.
+//!
+//! This mirrors the intent of the legacy C++ `Platform` interface
+//! (see docs/PRIOR_ART.md) with one deliberate change: `poll_events` is the
+//! only place backends need to bridge their native event model (X11's
+//! blocking `XNextEvent`, Wayland's callback-driven dispatch, Win32's message
+//! pump, macOS's event taps) into the common [`srdwm_core::Event`] queue -
+//! everything downstream of that is platform-independent.
+
+use srdwm_core::{Monitor, Rect, Window, WindowId};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum PlatformKind {
+    X11,
+    Wayland,
+    Windows,
+    MacOS,
+}
+
+impl PlatformKind {
+    pub fn name(self) -> &'static str {
+        match self {
+            PlatformKind::X11 => "x11",
+            PlatformKind::Wayland => "wayland",
+            PlatformKind::Windows => "windows",
+            PlatformKind::MacOS => "macos",
+        }
+    }
+}
+
+/// Chooses a backend the way the legacy `PlatformFactory` did: prefer
+/// Wayland when a compositor is reachable, fall back to X11, otherwise use
+/// the compile-time native backend on Windows/macOS.
+pub fn detect() -> PlatformKind {
+    #[cfg(target_os = "windows")]
+    {
+        return PlatformKind::Windows;
+    }
+    #[cfg(target_os = "macos")]
+    {
+        return PlatformKind::MacOS;
+    }
+    #[cfg(all(unix, not(target_os = "macos")))]
+    {
+        let wayland = std::env::var_os("WAYLAND_DISPLAY").is_some()
+            || std::env::var("XDG_SESSION_TYPE").map(|v| v == "wayland").unwrap_or(false);
+        if wayland {
+            PlatformKind::Wayland
+        } else {
+            PlatformKind::X11
+        }
+    }
+}
+
+#[derive(thiserror::Error, Debug)]
+pub enum PlatformError {
+    #[error("failed to connect to display server: {0}")]
+    ConnectionFailed(String),
+    #[error("another window manager is already running")]
+    AnotherWmRunning,
+    #[error("unsupported operation on this platform: {0}")]
+    Unsupported(&'static str),
+    #[error("{0}")]
+    Other(String),
+}
+
+pub type Result<T> = std::result::Result<T, PlatformError>;
+
+/// A platform backend: owns the connection to the display server and
+/// translates between real surfaces/windows and srdwm-core's `Window` state.
+pub trait Platform {
+    fn kind(&self) -> PlatformKind;
+
+    /// Blocks until at least one event is available (or a short timeout
+    /// elapses), then drains everything currently pending.
+    fn poll_events(&mut self) -> Result<Vec<srdwm_core::Event>>;
+
+    fn monitors(&mut self) -> Result<Vec<Monitor>>;
+
+    fn apply_geometry(&mut self, window: WindowId, geometry: Rect) -> Result<()>;
+    fn set_title(&mut self, window: WindowId, title: &str) -> Result<()>;
+    fn focus(&mut self, window: WindowId) -> Result<()>;
+    fn minimize(&mut self, window: WindowId) -> Result<()>;
+    fn restore(&mut self, window: WindowId) -> Result<()>;
+    fn close(&mut self, window: WindowId) -> Result<()>;
+
+    fn set_decorated(&mut self, window: WindowId, decorated: bool) -> Result<()>;
+    fn set_border_color(&mut self, window: WindowId, rgb: (u8, u8, u8)) -> Result<()>;
+    fn set_border_width(&mut self, window: WindowId, width: u32) -> Result<()>;
+
+    /// Redraws the decoration (titlebar + buttons) for `window`, e.g. after
+    /// a focus change or resize. No-op on platforms with native
+    /// decorations (Windows/macOS).
+    fn redraw_decoration(&mut self, window: WindowId, win: &Window, focused: bool) -> Result<()>;
+
+    fn grab_keyboard(&mut self) -> Result<()>;
+    fn ungrab_keyboard(&mut self) -> Result<()>;
+}