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

Exercise Library/Kneeling Hip Flexor Stretch/info.md

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+# Kneeling Hip Flexor Stretch
+
+Drop into a half-kneel and press the hips forward to open the front of the back
+hip. The go-to release for tight hip flexors after long hours of sitting or
+heavy squatting.
+
+- **Category:** Stretching
+- **Type:** Static stretch (isometric hold)
+- **Targets:** Hip flexors, quadriceps, front of the hip
+- **Prescription:** Hold 30 s per side, pressing the hips gently forward
+- **Defaults:** 2 × 30 s
+
+## Setup
+
+Kneel on one knee with the other foot planted flat in front, the front knee
+stacked over the ankle at about ninety degrees. Rest the hands on the front
+thigh. Pad the back knee if needed.
+
+## Execution
+
+1. Tuck the tailbone under to flatten the low back.
+2. Ease the hips forward until you feel a stretch across the front of the back
+   hip and thigh.
+3. Keep the trunk tall and the front shin vertical.
+4. Hold and breathe, then switch sides.
+
+## Cues
+
+- Squeeze the glute of the kneeling leg to deepen the hip-flexor stretch.
+- Tuck the tailbone first — don't just arch the low back forward.
+- Keep the front knee tracking over the foot, not caving inward.
+
+## Common Mistakes
+
+- Letting the front knee drift far past the toes.
+- Arching the low back instead of tucking the pelvis and pressing the hips.
+- Leaning the whole torso forward rather than shifting the hips.
+
+## Progression
+
+Hands on the front thigh → reach the same-side arm overhead and lean away for
+a deeper stretch → add a gentle back-foot lift toward the glute to bring in the
+quad.