Implement crouch and prone stances

Docs/MovementAndTimeScaleBaseline.md

@@ -116,6 +116,26 @@ Later movement work should separate:
 - injury/load/terrain modifiers;
 - skill, age, care history, and condition effects.
 
+## MVP Stance Movement
+
+The first stance pass supports crouch and prone as low-movement modes that keep
+the same real-distance philosophy as walking and sprinting.
+
+Current defaults:
+
+- crouch toggles with `C` or gamepad Right Shoulder;
+- prone toggles with `Z` or gamepad Left Shoulder;
+- crouch uses `55%` of the current movement speed;
+- prone uses `25%` of the current movement speed;
+- sprint cannot start while crouched or prone;
+- entering prone exits crouch so the player has one active low stance at a
+  time.
+
+The MVP prone state is replicated and affects movement speed. Dedicated prone
+animation, capsule resizing, crawl-specific collision, stealth/noise, and
+surface-specific crawl penalties are reserved for later animation and traversal
+passes.
+
 ## Movement Modifier Implementation
 
 MVP movement speed is now calculated as: