physics: implement Water Walk movement state tracking and surface clamping

SMSG_MOVE_WATER_WALK / SMSG_MOVE_LAND_WALK now correctly set/clear
WATER_WALK (0x00008000) in movementInfo.flags, ensuring the flag is
included in movement ACKs sent to the server.

In CameraController, when waterWalkActive_ is set and the player is
at or above the water surface (within 0.5 units), clamp them to the
water surface and mark as grounded — preventing water entry and allowing
them to walk across the water surface as the spell intends.

include/game/world_packets.hpp

@@ -396,6 +396,7 @@ enum class MovementFlags : uint32_t {
     FALLING             = 0x00001000,
     FALLINGFAR          = 0x00002000,
     FEATHER_FALL        = 0x00004000,  // Slow fall / Parachute
+    WATER_WALK          = 0x00008000,  // Walk on water surface
     SWIMMING            = 0x00200000,
     ASCENDING           = 0x00400000,
     CAN_FLY             = 0x00800000,