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/Arm Circles.info.md

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+# Arm Circles
+
+Big, slow circles of both arms together to warm the shoulders, wake up the
+rotator cuff, and open the chest before pressing or pulling.
+
+- **Category:** Mobility / warm-up
+- **Type:** Shoulder mobility
+- **Targets:** Deltoids, rotator cuff, upper back
+- **Prescription:** 10 slow backward circles, then 10 forward
+- **Defaults:** 2 × 10 bodyweight
+
+## Setup
+
+Stand tall, feet shoulder-width, arms relaxed at your sides.
+
+## Execution
+
+1. Reach both arms forward and up until they pass overhead.
+2. Carry them back and down behind you, tracing the biggest circle you can.
+3. Sweep them low and forward again to begin the next circle.
+4. Complete the set, then circle in the opposite direction.
+
+## Cues
+
+- Keep the arms long — draw the circle from the shoulder, not the elbow.
+- Stand tall; don't let the low back arch as the arms pass overhead.
+- Keep the tempo slow and even, breathing throughout.
+
+## Common Mistakes
+
+- Tiny, rushed circles instead of full, slow ones.
+- Shrugging the shoulders toward the ears at the top.
+- Arching the low back to force the arms higher.
+
+## Progression
+
+Small circles → full-range circles → slow both directions → pause briefly with
+the arms overhead each rep.