Add implicit animation widgets and motion gallery

[ExtraBinoss]

Summary

• add AnimatedOpacity and AnimatedContainer with interruption-safe retained transitions and reduced-motion behavior
• expose a user-implementable Curve trait plus nine reusable curve presets
• expand the Animation Laboratory to 20 examples covering implicit transitions, keyframes, orchestration, physics and composition
• add the animation guide, website lesson, runnable documentation examples and feature-gate checks

Verification after rebasing on main

• cargo clippy for argui-animation, argui-widgets and argui-widget-gallery with all targets and features
• cargo nextest for the three affected crates: 431 passed
• docs example suite: 15 passed
• website documentation tests: 4 passed
• shadcn catalogue contract test passed
• Chromium/WebGPU motion scenario passed on the private Wayland display

The complete quality gate also passed during implementation with 1,405 tests and every workspace coverage metric above 85%.

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