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/Romanian Deadlift/motion.json

@@ -0,0 +1,35 @@
+{
+  "name": "Romanian Deadlift",
+  "primary": 1,
+  "camera": {"zoom": 0.7},
+  "working": ["leg_r", "leg_l", "spine"],
+  "props": [
+    {"type": "bar", "at": ["hand_r", "hand_l"], "halfLen": 26, "plateR": 13}
+  ],
+  "frames": [
+    {
+      "hold": 0.4, "tween": 0.9,
+      "root": {"pos": [90, 92], "pitch": 68},
+      "spine": [0, -6],
+      "neck": 5, "head": -62,
+      "shoulder_r": 66, "elbow_r": 4,
+      "shoulder_l": 66, "elbow_l": 4,
+      "hip_r": 112, "knee_r": 12, "ankle_r": 12,
+      "hip_l": 112, "knee_l": 12, "ankle_l": 12,
+      "pins": {"foot_r": [158, 147], "foot_l": [162, 149],
+               "hand_r": [167, 120], "hand_l": [171, 122]}
+    },
+    {
+      "hold": 0.4, "tween": 1.4,
+      "root": {"pos": [158, 64], "pitch": 10},
+      "spine": [0, 0],
+      "neck": 2, "head": -8,
+      "shoulder_r": 10, "elbow_r": 3,
+      "shoulder_l": 10, "elbow_l": 3,
+      "hip_r": 8, "knee_r": 8, "ankle_r": 0,
+      "hip_l": 8, "knee_l": 8, "ankle_l": 0,
+      "pins": {"foot_r": [158, 147], "foot_l": [162, 149],
+               "hand_r": [169, 38], "hand_l": [173, 40]}
+    }
+  ]
+}