Add 17 warm-up, stretch, and mobility exercises with new starter splits

The library grows to 64 exercises: arm circles, torso twist, leg
swings, hip and neck mobility, marching, calf raises, and a full
stretching set (forward fold, quad, calf, chest, triceps, hip flexor,
butterfly, cobra, child's pose), each with an authored motion rig and
reference page. Eight new starter splits join the catalog — Upper and
Lower Body Warm-Up, Morning Wake-Up, Morning Mobility, Full Body
Stretch, Evening Stretch, Free Weight Basics, and Full Body Machines —
regenerated deterministically at split schema v3 with fixed ULIDs.

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

Workouts/Resources/ExerciseMotions/Torso Twist.info.md

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+# Torso Twist
+
+Relaxed standing rotation from side to side, letting the arms swing freely, to
+warm the spine and obliques before lifting.
+
+- **Category:** Mobility / warm-up
+- **Type:** Spinal rotation
+- **Targets:** Obliques, thoracic spine, lower back
+- **Prescription:** 10 easy twists to each side, arms loose
+- **Defaults:** 2 × 10 bodyweight
+
+## Setup
+
+Stand with feet shoulder-width, knees soft, arms hanging loose.
+
+## Execution
+
+1. Turn the trunk gently to one side, letting the arms swing across the body.
+2. Let that momentum carry you back through center to the other side.
+3. Keep a smooth, relaxed rhythm turning side to side.
+
+## Cues
+
+- Let the arms be floppy — they trail the twist, they don't drive it.
+- Rotate from the waist and mid-back while the hips stay mostly forward.
+- Keep the neck loose and let the head follow the shoulders.
+
+## Common Mistakes
+
+- Yanking the arms to force more rotation.
+- Letting the feet peel off the floor as you turn.
+- Twisting so hard the low back pinches.
+
+## Progression
+
+Small easy twists → fuller rotation → slow, controlled turns with a brief pause
+at each end.