Ship Android numeric text showcase

[AmatoGiulio]

What changed

• replaces the example app with the minimal Android numeric-transition showcase
• adds native icon buttons with press-and-hold increment and decrement
• adds the continuous showcase sequence and removes the old lab/reference screens
• expands the public README and includes a normal-speed 15-second Pixel demo

Why

This prepares the first public release and closes the loop on the Android counterpart to SwiftUI's numeric text transition.

Validation

• yarn check
• yarn prepare
• npm pack --dry-run