WASD movement for the player

While W/A/S/D are held, the controlled creature gets a plain walk order every
engine tick toward its position plus the pressed screen direction, as the
mouse does while its button is held (every mouse event re-sends a walk to the
cursor point); releasing sends the target-less stop order of a released mouse
button. W/A/S/D key events are swallowed before the hotkeys see them; their
old actions move to G (action slot 3), V (pick up nearby) and K (save game
window) for now, configurable in [wasd].

docs/wasd.md: order format, measured mouse behaviour, hotkey map.
docs/todo.md: better pathfinder; final bindings for the moved hotkeys.
re::ConfigString for string settings.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

CMakeLists.txt

@@ -33,6 +33,7 @@ add_library(sacred_re SHARED
     src/mods/borderless.cpp
     src/mods/widescreen.cpp
     src/mods/hud.cpp
+    src/mods/wasd.cpp
 )
 target_include_directories(sacred_re PRIVATE src)
 target_compile_options(sacred_re PRIVATE ${SACRED_RE_CXX_FLAGS})