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/info.md

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+# Romanian Deadlift
+
+The stretch-focused hip hinge — push the hips back with soft knees until
+the hamstrings stop you, then stand back up. Barbell or dumbbells.
+
+- **Category:** Main circuit
+- **Type:** Free-weight hinge (barbell or dumbbells)
+- **Targets:** Hamstrings, glutes, spinal erectors
+- **Prescription:** 3 × 8–10
+
+## Setup
+
+Stand tall holding the bar (or dumbbells) at the hips, feet hip-width,
+knees unlocked. Set your shoulders back and brace.
+
+## Execution
+
+1. Push the hips straight back and let the weight slide down the thighs,
+   back flat, knees only slightly bent.
+2. Stop when the hamstrings are fully stretched — usually just below the
+   knees — without the back rounding.
+3. Drive the hips forward to stand tall again.
+
+## Cues
+
+- The hips travel backward, not down — this is not a squat.
+- Weight stays close to the legs the whole rep.
+- Feel the hamstrings load on the way down; that stretch is the exercise.
+
+## Common Mistakes
+
+- Bending the knees more as the weight lowers, turning it into a deadlift.
+- Rounding the back to reach the floor — depth comes from the hips.
+- Rushing the lowering phase.
+
+## Progression
+
+Add weight once the full stretch is reached with a flat back every rep;
+single-leg RDLs for balance and asymmetry work.