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, 79 insertions, 0 deletions

diff --git a/crates/config/src/value.rs b/crates/config/src/value.rs
new file mode 100644
index 0000000..e59ccb9
--- /dev/null
+++ b/crates/config/src/value.rs
@@ -0,0 +1,79 @@
+use mlua::Value;
+
+/// A scalar (or short list of scalars) read from `srd.set(key, value)`.
+/// Deliberately simpler than the legacy C++ `LuaConfigValue`, which also had
+/// `Table`/`Function` variants that, per docs/PRIOR_ART.md, were never
+/// actually exercised by the flat dotted-key config style the example
+/// configs (and `docs/DEFAULTS.md`) actually use.
+#[derive(Debug, Clone, PartialEq)]
+pub enum ConfigValue {
+    String(String),
+    Number(f64),
+    Bool(bool),
+    List(Vec<String>),
+}
+
+impl ConfigValue {
+    pub fn from_lua(value: &Value) -> Option<Self> {
+        match value {
+            Value::String(s) => Some(ConfigValue::String(s.to_str().ok()?.to_string())),
+            Value::Number(n) => Some(ConfigValue::Number(*n)),
+            Value::Integer(i) => Some(ConfigValue::Number(*i as f64)),
+            Value::Boolean(b) => Some(ConfigValue::Bool(*b)),
+            Value::Table(t) => {
+                let mut items = Vec::new();
+                for pair in t.clone().sequence_values::<String>() {
+                    items.push(pair.ok()?);
+                }
+                Some(ConfigValue::List(items))
+            }
+            _ => None,
+        }
+    }
+
+    pub fn as_str(&self) -> Option<&str> {
+        match self {
+            ConfigValue::String(s) => Some(s),
+            _ => None,
+        }
+    }
+
+    pub fn as_f64(&self) -> Option<f64> {
+        match self {
+            ConfigValue::Number(n) => Some(*n),
+            _ => None,
+        }
+    }
+
+    pub fn as_bool(&self) -> Option<bool> {
+        match self {
+            ConfigValue::Bool(b) => Some(*b),
+            _ => None,
+        }
+    }
+
+    pub fn as_list(&self) -> Option<&[String]> {
+        match self {
+            ConfigValue::List(l) => Some(l),
+            _ => None,
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use mlua::Lua;
+
+    #[test]
+    fn converts_string_number_bool_and_list() {
+        let lua = Lua::new();
+        assert_eq!(ConfigValue::from_lua(&lua.load("'hi'").eval().unwrap()), Some(ConfigValue::String("hi".into())));
+        assert_eq!(ConfigValue::from_lua(&lua.load("42").eval().unwrap()), Some(ConfigValue::Number(42.0)));
+        assert_eq!(ConfigValue::from_lua(&lua.load("true").eval().unwrap()), Some(ConfigValue::Bool(true)));
+        assert_eq!(
+            ConfigValue::from_lua(&lua.load("{'1','2','3'}").eval().unwrap()),
+            Some(ConfigValue::List(vec!["1".into(), "2".into(), "3".into()]))
+        );
+    }
+}