Add a Running exercise with an animated stride figure

Stance and flight keyframes with single-frame foot pins for the strikes,
forward lean, opposite-arm drive, and daylight under both feet in flight —
distinct from Cardio's high-knee march, which stays as is.

Claude-Session: https://claude.ai/code/session_01PKptrgbx74peTwHGRxBojv

EXERCISE-LIBRARY.md

@@ -15,7 +15,7 @@ Data flows one way: you edit the **source**, run `render.py --export`, and the
 **bundle** is overwritten. Never hand-edit anything under
 `Workouts/Resources/ExerciseMotions/` — it is regenerated.
 
-As of this writing the catalog is **64 exercises**.
+As of this writing the catalog is **65 exercises**.
 
 ```
 Exercise Library/                         ← authoring source (repo root)
@@ -23,7 +23,7 @@ Exercise Library/                         ← authoring source (repo root)
 ├── skeleton.json                         ← shared rig: bone lengths + joint ROM
 ├── render.py   kinematics.py             ← the render pipeline + the FK math
 ├── contact-sheet.png  demo-sheet.png     ← generated QA sheets
-└── <Exercise Name>/                      ← one folder per exercise, 64 of them
+└── <Exercise Name>/                      ← one folder per exercise, 65 of them
     ├── info.md        ← AUTHORED: the reference page
     ├── motion.json    ← AUTHORED: the rig script (canonical visual source)
     ├── frames/frame-N.svg   ← generated: one SVG per key frame
@@ -35,8 +35,8 @@ Exercise Library/                         ← authoring source (repo root)
           ▼
 Workouts/Resources/ExerciseMotions/       ← bundle (flat, unique basenames)
 ├── skeleton.json
-├── <Exercise Name>.motion.json           ← 64
-└── <Exercise Name>.info.md               ← 64
+├── <Exercise Name>.motion.json           ← 65
+└── <Exercise Name>.info.md               ← 65
 
           │  Bundle.main lookups
           ▼
@@ -208,7 +208,7 @@ three kinds of file:
 
 The generated SVGs and GIFs do **not** ship — the app renders the figure live
 from the rig instead. The rename to `<Name>.motion.json` / `<Name>.info.md` is
-what lets Xcode's flat resource copy hold all 64 without collision.
+what lets Xcode's flat resource copy hold all 65 without collision.
 
 ## The app-side consumers (`Workouts/ExerciseFigure/`)