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/Neck Rolls.info.md

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+# Neck Rolls
+
+Slow, gentle circles of the head to loosen the neck and upper spine before
+training or after a long day at a desk.
+
+- **Category:** Mobility / warm-up
+- **Type:** Neck mobility
+- **Targets:** Cervical spine, upper trapezius, levator scapulae
+- **Prescription:** 3–5 slow circles in each direction, stopping short of any pinch
+- **Defaults:** 2 × 5 bodyweight
+
+## Setup
+
+Stand tall, feet hip-width, shoulders relaxed and down, arms hanging quietly at
+your sides.
+
+## Execution
+
+1. Drop the chin toward the chest.
+2. Roll one ear down toward that shoulder, then carry the head back into a
+   gentle look-up.
+3. Bring the other ear toward the far shoulder and back down to the chest — one
+   smooth, continuous circle.
+4. Finish the circles, then reverse the direction.
+
+## Cues
+
+- Move slowly — a single circle should take several seconds.
+- Keep the shoulders down and still; only the head travels.
+- Ease off the moment you feel a pinch, especially at the look-up.
+
+## Common Mistakes
+
+- Whipping the head around quickly.
+- Shrugging a shoulder up to meet the ear.
+- Forcing the range at the top of the circle.
+
+## Progression
+
+Small pain-free arcs → full slow circles → add a brief pause at each ear and at
+the chin tuck.