Fix NPCs walking backwards

Avatars called Godot's look_at(), which points the node's -Z axis at the target,
but the rig's face (eyes/brows/mouth/focus disc, and the Blender successor) is on
+Z. Every walking NPC therefore faced away from its movement direction.

- add the pure yaw_towards(direction) helper (+Z forward) and use it in both
  movement paths and the reaction face-turn
- fix the head-turn offset to the same +Z convention (basis.z)
- test +Z/+X/-Z/arbitrary directions; recapture targeting screenshot
- TODO bug item moved to DONE, MEMORY records the convention

Headless 209/209.

src/test/tests/test_npc_character.gd

@@ -55,6 +55,23 @@ func test_idle_styles_are_known() -> bool:
 		ok = ok and style in allowed
 	return TestUtils.check(ok, "idle styles stay in the catalogue")
 
+func test_yaw_faces_the_movement_direction() -> bool:
+	var a := TestUtils.check(is_zero_approx(NpcAvatar.yaw_towards(Vector3(0, 0, 1))),
+		"+Z is the neutral facing")
+	var x_dir := NpcAvatar.yaw_towards(Vector3(1, 0, 0))
+	var b := TestUtils.check(is_equal_approx(x_dir, PI / 2.0),
+		"+X turns a quarter turn")
+	var back := NpcAvatar.yaw_towards(Vector3(0, 0, -1))
+	var c := TestUtils.check(absf(absf(back) - PI) < 0.001, "-Z is a half turn")
+	var direction := Vector3(0.4, 0, -0.9).normalized()
+	var yaw := NpcAvatar.yaw_towards(direction)
+	var facing := Basis(Vector3.UP, yaw) * Vector3(0, 0, 1)
+	var d := TestUtils.check(facing.dot(direction) > 0.999,
+		"the avatar's +Z face points along the walk direction")
+	var e := TestUtils.check(is_zero_approx(NpcAvatar.yaw_towards(Vector3.ZERO)),
+		"zero direction falls back to neutral")
+	return a and b and c and d and e
+
 func test_queue_slots_are_spaced_and_unique() -> bool:
 	var seen := {}
 	var ok := true