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/Back Extension/info.md

@@ -0,0 +1,39 @@
+# Back Extension
+
+The roman-chair hinge — hips anchored on the pad, fold forward, then raise
+the trunk back in line with the legs.
+
+- **Category:** Main circuit
+- **Type:** Machine-based back extension (hip hinge)
+- **Targets:** Spinal erectors, glutes, hamstrings
+- **Prescription:** 3 × 10–15
+
+## Setup
+
+Set the pad so its top edge sits at the crease of your hips — you must be
+able to fold freely over it. Hook your heels under the ankle rollers,
+cross your arms over your chest.
+
+## Execution
+
+1. Fold forward over the pad with a controlled, mostly-flat back.
+2. Lower until the hamstrings stop you, without going slack.
+3. Squeeze glutes and back to raise the trunk until it lines up with the
+   legs — no higher.
+
+## Cues
+
+- Movement comes from the hips with the back held long.
+- Rise to a straight body line, not into an arch.
+- Slow down; momentum does nothing here.
+
+## Common Mistakes
+
+- Pad set too high, blocking the hinge.
+- Hyperextending past the body line at the top.
+- Swinging up with momentum.
+
+## Progression
+
+Slow the lowering phase, add a pause at the top, then hold a plate to the
+chest for load.