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/Standing Quad Stretch.info.md

@@ -0,0 +1,42 @@
+# Standing Quad Stretch
+
+Balance on one leg, catch the opposite foot, and draw the heel toward the seat
+to lengthen the front of the thigh. A quick, equipment-free way to loosen the
+quads after running or squatting.
+
+- **Category:** Stretching
+- **Type:** Static stretch (isometric hold)
+- **Targets:** Quadriceps (front of the thigh), hip flexors
+- **Prescription:** Hold 30 s per side, keeping the knees together
+- **Defaults:** 2 × 30 s
+
+## Setup
+
+Stand tall next to a wall for balance if needed. Shift the weight onto one leg
+and bend the other knee, reaching back with the same-side hand to hold the
+foot or ankle.
+
+## Execution
+
+1. Draw the heel gently toward the glute until the front of the thigh
+   lengthens.
+2. Keep the knees pressed together and the thigh pointing straight down.
+3. Stand tall — tuck the tailbone so the low back doesn't arch.
+4. Hold and breathe, then lower the foot and switch sides.
+
+## Cues
+
+- Keep the bent knee pointing at the floor, not flaring out to the side.
+- Squeeze the glute of the stretching leg to add a hip-flexor stretch.
+- Reach the free arm out or up for balance instead of gripping a wall hard.
+
+## Common Mistakes
+
+- Letting the knee drift forward and the back arch to reach further.
+- Pulling the heel hard into the glute and cranking the knee.
+- Hunching or leaning far forward to grab the foot.
+
+## Progression
+
+Hold a wall or chair lightly → free-standing for balance → add a slight
+forward hinge of the trunk to deepen the hip-flexor component.