feat(vagrant-skeleton): Subterrain-style lower-body control model

Add lower_control virtual bone (reparents legs for independent orientation).
Update look_at fields: body uses "soft" target with slerp=6, head uses
"aim" target with instant snap and ±60° clamp. Rename walk.anim.json to
walk_fwd_lower.anim.json and add walk_back_lower, walk_left_lower,
walk_right_lower anims ported from substrate. Port HandleAnimationLower:
signed-angle sector selection (±45°/±135°) drives 4-directional walk anim
choice; wedge clamp sets lower_control rotation. Separate aim/soft look
targets both track raw mouse for now.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

animations/walk_back_lower.anim.json

@@ -0,0 +1,28 @@
+{
+  "id": "walk_back_lower",
+  "track": "lower",
+  "duration": 0.8,
+  "loop": true,
+  "keyframes": [
+    { "t": 0.0,
+      "leg_l":  {"scl": [-1,  0]},  "leg_r":  {"scl": [ 1,  0]},
+      "foot_l": {"pos": [0, 0]},    "foot_r": {"pos": [0, 0]}
+    },
+    { "t": 0.2,
+      "leg_l":  {"scl": [-1, -1]},  "leg_r":  {"scl": [ 1,  1]},
+      "foot_l": {"pos": [-0.044,  25.47]},  "foot_r": {"pos": [0.249,  25.85]}
+    },
+    { "t": 0.4,
+      "leg_l":  {"scl": [-1,  0]},  "leg_r":  {"scl": [ 1,  0]},
+      "foot_l": {"pos": [0, 0]},    "foot_r": {"pos": [0, 0]}
+    },
+    { "t": 0.6,
+      "leg_l":  {"scl": [-1,  1]},  "leg_r":  {"scl": [ 1, -1]},
+      "foot_l": {"pos": [ 0.044, -25.47]},  "foot_r": {"pos": [-0.249, -25.85]}
+    },
+    { "t": 0.8,
+      "leg_l":  {"scl": [-1,  0]},  "leg_r":  {"scl": [ 1,  0]},
+      "foot_l": {"pos": [0, 0]},    "foot_r": {"pos": [0, 0]}
+    }
+  ]
+}