feat: procedural human standing motion engine & pose reset options (Dynamic A-Pose vs Rigid T-Pose)

[Ming2k8-Coder]

Description

This PR introduces a real-time procedural standing animation engine and flexible pose reset options:

• Procedural Human Kinematics: Synthesizes real-time breathing, postural sway, center-of-mass weight shifts, and micro eye saccades.
• Natural A-Pose Standing: Replaces the default rigid T-pose with a natural human A-pose stance (arms lowered ~70°, relaxed elbows, curved fingers).
• Pose Reset Selector: Adds a dropdown next to Reset Pose allowing users to switch between Dynamic A-Pose (natural standing) and Rigid T-Pose (raw rest pose).

Diff Size

~29 lines modified in index.html.