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/Overhead Triceps Stretch.info.md

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+# Overhead Triceps Stretch
+
+Reach one arm overhead and let the hand drop behind the neck, then use the
+other hand to ease the elbow back. A simple way to lengthen the triceps and
+open the underside of the shoulder between pressing sets.
+
+- **Category:** Stretching
+- **Type:** Static stretch (isometric hold)
+- **Targets:** Triceps (back of the upper arm), lats, shoulder
+- **Prescription:** Hold 30 s per side, easing the elbow back gently
+- **Defaults:** 2 × 30 s
+
+## Setup
+
+Stand tall with the feet hip-width. Raise one arm overhead and bend the elbow
+so the hand drops down behind the neck.
+
+## Execution
+
+1. Reach the free hand up and rest it on the raised elbow.
+2. Draw the elbow gently back and toward the midline until the triceps
+   lengthens.
+3. Let the trunk lean a touch away from the raised arm to deepen the stretch.
+4. Hold and breathe, then release and switch arms.
+
+## Cues
+
+- Ease the elbow back, not down — coax it, never wrench it.
+- Keep the ribs from flaring by drawing them softly down.
+- Lengthen tall through the spine before adding the side lean.
+
+## Common Mistakes
+
+- Cranking the head forward to meet the arm instead of dropping the hand behind.
+- Flaring the ribs and arching the low back to fake more range.
+- Shrugging the working shoulder up toward the ear.
+
+## Progression
+
+Walk the hand further down the spine → add a gentle side bend away from the
+raised arm → hold a strap between the top and bottom hands to link both
+shoulders.