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.

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diff --git a/legacy-cpp/DEPENDENCIES.md b/legacy-cpp/DEPENDENCIES.md
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+# SRDWM Dependencies
+
+This file lists the required and optional dependencies per platform. Package names are provided for popular distros. If your distro is not listed, install equivalent packages and ensure `pkg-config` can find them.
+
+## Common Tools
+
+- C++17 compiler (gcc/clang or MSVC)
+- CMake ≥ 3.20 recommended
+- Ninja (recommended) or Make
+- pkg-config
+- Git
+- Lua 5.4 + development headers
+
+## Linux (X11)
+
+Required libraries:
+- X11: x11, xrandr, xinerama, xfixes, xcursor
+- XCB: xcb, xcb-keysyms, xcb-icccm, xcb-ewmh, xcb-randr
+- Optional: Xft for font rendering (`xft`)
+
+Distro packages:
+- Ubuntu/Debian: `libx11-dev libxrandr-dev libxinerama-dev libxfixes-dev libxcursor-dev libxcb1-dev libxcb-keysyms1-dev libxcb-icccm4-dev libxcb-ewmh-dev libxcb-randr0-dev libxft-dev`
+- Fedora: `libX11-devel libXrandr-devel libXinerama-devel libXfixes-devel libXcursor-devel libxcb-devel xcb-util-keysyms-devel xcb-util-wm-devel xcb-util-renderutil-devel xcb-util-image-devel libXft-devel`
+- Arch: `libx11 libxrandr libxinerama libxfixes libxcursor libxcb xcb-util xcb-util-keysyms xcb-util-wm libxft`
+- openSUSE: `libX11-devel libXrandr-devel libXinerama-devel libXfixes-devel libXcursor-devel libxcb-devel xcb-util-keysyms-devel xcb-util-wm-devel libXft-devel`
+- Alpine: `libx11-dev libxrandr-dev libxinerama-dev libxfixes-dev libxcursor-dev libxcb-dev xcb-util-keysyms-dev xcb-util-wm-dev libxft-dev`
+
+## Linux (Wayland)
+
+Wayland can be built in two modes:
+- Stub backend (default): builds without linking to full wlroots at runtime. Set `-DUSE_WAYLAND_STUB=ON`.
+- Real wlroots backend: full compositor link. Requires wlroots and additional libs.
+
+Required packages for real backend:
+- Wayland: `wayland-client`, `wayland-protocols`
+- wlroots: `wlroots`
+- Others: `pixman-1`, `libxkbcommon`, `libdrm`, `EGL/Mesa`, `gbm`, `libinput`, `seatd`, `udev`
+
+Distro packages:
+- Ubuntu/Debian: `libwayland-dev wayland-protocols libwlroots-dev libpixman-1-dev libxkbcommon-dev libdrm-dev libegl1-mesa-dev libgbm-dev libinput-dev libudev-dev libseat-dev`
+- Fedora: `wayland-devel wayland-protocols-devel wlroots-devel pixman-devel libxkbcommon-devel libdrm-devel mesa-libEGL-devel mesa-libgbm-devel libinput-devel seatd-devel systemd-devel`
+- Arch: `wayland wayland-protocols wlroots pixman libxkbcommon libdrm egl-wayland libgbm libinput seatd`
+- openSUSE: `wayland-devel wayland-protocols-devel wlroots-devel pixman-devel libxkbcommon-devel libdrm-devel Mesa-libEGL-devel Mesa-libgbm-devel libinput-devel seatd-devel`
+- Alpine: `wayland-dev wayland-protocols wlroots-dev pixman-dev libxkbcommon-dev libdrm-dev mesa-egl mesa-gbm libinput-dev seatd-dev`
+
+## Windows
+
+- Visual Studio 2019+ with C++ toolset or MSVC Build Tools
+- CMake and Ninja recommended
+- Lua 5.4 via vcpkg: `vcpkg install lua:x64-windows`
+- Windows SDK (10.0.19041.0+)
+
+Use: `scripts\install_deps_windows_vcpkg.ps1`
+
+## macOS
+
+- Xcode Command Line Tools
+- Homebrew packages: `cmake ninja lua pkg-config git`
+
+Use: `bash scripts/install_deps_macos.sh`
+
+## Notes
+
+- If `pkg-config` cannot find a dependency, ensure the `-dev` (headers) package is installed and `PKG_CONFIG_PATH` is set correctly.
+- You can disable Wayland entirely with `-DENABLE_WAYLAND=OFF` if building for X11 only.
+- The default configuration enables Wayland with a stub backend for broader compatibility. Switch to the real backend with `-DUSE_WAYLAND_STUB=OFF` once `wlroots` and related deps are available.