Add the hinge pattern: Deadlift, RDL, Glute Bridge, Hip Thrust, Back Extension

The biggest hole in the coverage matrix - the hinge had zero entries in
any modality. Deadlift and Romanian Deadlift are the library's first
standing (camera.zoom) and first barbell motions.

Claude-Session: https://claude.ai/code/session_01HJDQQDA9QdP8zByg43H5v3

Exercise Library/Hip Thrust/motion.json

@@ -0,0 +1,38 @@
+{
+  "name": "Hip Thrust",
+  "primary": 2,
+  "working": ["leg_r", "leg_l"],
+  "props": [
+    {"type": "scene", "shapes": [
+      {"kind": "line", "pts": [[50, 122], [88, 122]], "w": 9, "depth": 8},
+      {"kind": "line", "pts": [[68, 127], [68, 150]], "w": 5}
+    ]},
+    {"type": "bar", "at": ["hand_r", "hand_l"], "halfLen": 26, "plateR": 13}
+  ],
+  "frames": [
+    {
+      "hold": 0.4, "tween": 0.8,
+      "root": {"pos": [150, 136], "pitch": -75},
+      "spine": [0, 6],
+      "neck": 42, "head": -4,
+      "shoulder_r": 4, "elbow_r": 8,
+      "shoulder_l": 4, "elbow_l": 8,
+      "hip_r": 78, "knee_r": 88, "ankle_r": 5,
+      "hip_l": 78, "knee_l": 88, "ankle_l": 5,
+      "pins": {"foot_r": [203, 147], "foot_l": [207, 149],
+               "hand_r": [124, 127], "hand_l": [128, 129]}
+    },
+    {
+      "hold": 0.9, "tween": 1.0,
+      "root": {"pos": [152, 114], "pitch": -89},
+      "spine": [0, 2],
+      "neck": 48, "head": -4,
+      "shoulder_r": 4, "elbow_r": 8,
+      "shoulder_l": 4, "elbow_l": 8,
+      "hip_r": 24, "knee_r": 92, "ankle_r": 5,
+      "hip_l": 24, "knee_l": 92, "ankle_l": 5,
+      "pins": {"foot_r": [203, 147], "foot_l": [207, 149],
+               "hand_r": [126, 111], "hand_l": [130, 113]}
+    }
+  ]
+}