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>

docs/todo.md

@@ -0,0 +1,30 @@
+# To do
+
+## Better pathfinder (mouse clicks and monsters)
+
+Replace the game's pathfinding with a more reliable and efficient one, for
+click-to-move and for monster AI. What we know so far:
+
+- Walk orders are `cEvent_creature` command 4 with a ground target (+0x18/+0x1c),
+  sent through `cEngine::SendCreatureOrder` (`0x6170d0`), which snaps the
+  target to a walkable point (`0x616b70`) and hands it to
+  `cCreature::ReceiveOrder` (`0x52e400`, vf18, ~6400 decompiled lines).
+- Far targets (~2000 world units) are rejected or followed badly.
+- Still to map: the path search itself, the sector/tile collision data it
+  reads, and how monster AI issues its moves.
+
+## Key bindings moved for WASD movement (temporary)
+
+W, A and S were game hotkeys. With WASD movement (`src/mods/wasd.cpp`) their
+actions moved to other keys for now; choose final bindings (and update the
+in-game texts/tooltips that name the old keys):
+
+| Key | Action | Temporary key (`[wasd]` in sacred_re.ini) |
+|---|---|---|
+| W | action slot 3 (`cEngine::UseActionSlot`, like Q/E/R/B/Space) | G (`ActionKeyW`) |
+| A | pick up items nearby (creature event 0x113) | V (`ActionKeyA`) |
+| S | save game window (`UI_WND_SAVEGAME`) | K (`ActionKeyS`) |
+| D | unbound | - |
+
+Globally bound letters (engine `0x618130`, UI manager `0x757120`): A B C E F
+H I L M N O P Q R S T W, Space, ^, digits. Individual windows may use more.