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, 20 insertions, 0 deletions

diff --git a/legacy-cpp/README.md b/legacy-cpp/README.md
new file mode 100644
index 0000000..054756a
--- /dev/null
+++ b/legacy-cpp/README.md
@@ -0,0 +1,20 @@
+# srdwm (legacy C++ prototype)
+
+This directory holds the original C++ prototype of srdwm, preserved for reference
+during the Rust rewrite. It is no longer built or maintained - see the repository
+root for the active Rust implementation.
+
+Status when archived (see `docs/IMPLEMENTATION_STATUS.md` and the root
+`docs/PRIOR_ART.md` porting notes for detail):
+
+- X11 backend: partially functional (reparenting decorations, EWMH atoms, no
+  drag/resize, hardcoded titlebar width, RandR monitor bug).
+- Windows backend: partially functional (DWM border color, global hooks, Win32
+  window ops); no virtual desktop support.
+- Wayland backend: architecture only - wlroots object graph created but no
+  listeners ever wired up, no windows are actually managed.
+- macOS backend: stub except Accessibility permission request and CGDisplay
+  monitor enumeration.
+- Lua config (`lua_manager.cc`): functional for scalar config get/set; key
+  bindings stored the combo string but not the actual Lua closure; several
+  `srd.*` functions (`spawn`, `load`, `notify`) were logging-only placeholders.