feat(client): follow-facing — body commits to path, mouse drives layered head/torso look-at (T-1088)

During an RMB path-follow the body yaw locks to the leg direction (rig
commit_body_to_motion, from interpolated velocity — immune to SetFacing
interleaving between throttled steps). The mouse instead drives a layered
look-at: LookAtModifier3D pair on Head (±70°) + spine_02 (±30° torso twist
for looking far lateral/behind — past their sum the character physically
cannot look further without turning). Forward axis measured from the
armature rest pose (+Z), not guessed. Influence fades in/out on follow
start/end. Pure client presentation per D-249; SetFacing still rides the
wire, so the server vision cone follows the mouse while walking — the
character looks where the player points.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

client/scripts/sandbox/sandbox_constants.gd

@@ -121,6 +121,17 @@ const TINTS := {
 ## Ground-plane deadzone (m) for the mouse facing provider — mirrors the 2D jitter guard.
 const MOUSE_AIM_DEADZONE_M := 0.1
 
+# -- Head/torso look-at during path-follow (T-1088 follow-facing) ---------------------
+
+## Head bone yaw limit (deg) for the mouse look-at while the body walks a path.
+const HEAD_LOOK_HEAD_LIMIT_DEG := 70.0
+## Torso (spine_02) contribution limit (deg) — engages when looking far lateral/behind.
+const HEAD_LOOK_TORSO_LIMIT_DEG := 30.0
+## Look-at influence fade in/out (s) on follow start/end.
+const HEAD_LOOK_FADE_S := 0.25
+## Aim point height above the mouse ground hit (m) — roughly eye level.
+const HEAD_LOOK_EYE_HEIGHT_M := 1.55
+
 # -- Path preview / click-to-move (T-1088 live-session item 2) ------------------------
 
 ## Move-here marker fill on the hovered destination subtile — distinct accent,