fix: stop feet sliding at gait phase transitions (reach, not ground-lock)

The closing / settling / about-face steps of the traveling gait moves
declared their feet `ground-lock: feet` (planted) while the feet were
actually relocating — closing home, stepping the trailing foot in, pivoting
through a 180° about-face, or coming together in a waltz close. The grounding
solver then reported the feet as planted-yet-drifting, up to ~0.95m
(walk-cycle about-face), ~0.74m (walk-cycle arrival), and 0.13-0.21m across
box-step / grapevine / chassé / waltz-box.

Declare those relocating feet as `reach: foot_left/right floor` instead: the
feet step to the floor at their settling position, which is what actually
happens, so the false planted-drift disappears and the motion is unchanged.

Only steps where the feet genuinely move are converted — a stationary
plié/lower keeps `ground-lock: feet`, and the deliberately mid-move
about-face turn's relocation is now honest.

Eval: 1559/1559 checks on both shipped proportions, 0 clamp warnings,
constraint warnings 151 -> 146; the large phase-transition foot-drift
warnings are gone. 397/397 tests pass.

Author: a-baran-orhan

spec/examples/box-step.posecode

@@ -44,7 +44,8 @@ posecode exercise "Box step"
     ankle_right: plantarflex 0
     shoulders: flex 0
     travel: 0 0
-    ground-lock: feet
+    reach: foot_left floor
+    reach: foot_right floor
     cue "Close the left foot and settle over both feet, completing the square"
 
   repeat 4

spec/examples/chasse.posecode

@@ -67,7 +67,8 @@ posecode exercise "Chassé"
     shoulders: abduct 0
     elbows: flex 0
     travel: 0 0
-    ground-lock: feet
+    reach: foot_left floor
+    reach: foot_right floor
     cue "Close home over both feet and let the momentum resolve"
 
   repeat 4

spec/examples/grapevine.posecode

@@ -72,7 +72,8 @@ posecode exercise "Grapevine"
     hip_right: abduct 0
     shoulders: abduct 0
     travel: 0 0
-    ground-lock: feet
+    reach: foot_left floor
+    reach: foot_right floor
     cue "Close the left foot under the body and settle the phrase"
 
   repeat 3

spec/examples/walk-cycle.posecode

@@ -28,7 +28,8 @@ posecode exercise "Walk & turn"
     shoulders: flex 0
     turn: 180
     travel: 0 0.66
-    ground-lock: feet
+    reach: foot_left floor
+    reach: foot_right floor
     cue "Plant and turn a half-turn to face back the way you came"
 
   step "Walk back" 0.7s flow:
@@ -48,7 +49,8 @@ posecode exercise "Walk & turn"
     shoulders: flex 0
     turn: 360
     travel: 0 0
-    ground-lock: feet
+    reach: foot_left floor
+    reach: foot_right floor
     cue "Arrive home and turn to face front again"
 
   repeat 2

spec/examples/waltz-box.posecode

@@ -28,7 +28,8 @@ posecode stretch "Waltz box step"
     knee_left: flex 0
     ankles: plantarflex 0
     travel: -0.2 0.2
-    ground-lock: feet
+    reach: foot_left floor
+    reach: foot_right floor
     cue "Close the right foot to the left and lower softly through count three"
 
   step "4 - left foot back" 0.95s flow:
@@ -57,7 +58,8 @@ posecode stretch "Waltz box step"
     shoulders: abduct 0
     elbows: flex 0
     travel: 0 0
-    ground-lock: feet
+    reach: foot_left floor
+    reach: foot_right floor
     cue "Close the left foot and lower with control to complete the six-count box"
 
   repeat 3