Add smooth-movement plugin

[notliad]

Summary

Origin: https://github.com/notliad/df-smooth-movement

Adds the smooth-movement plugin, which visually interpolates movement.

The plugin animates:

• creatures and multi-tile creature sprites
• hauled items
• vehicles
• associated status and designation icons
• movement on visible lower z-levels when enabled

All changes are rendering-only. The plugin does not modify simulation timing, gameplay state, or save data.

Features

• smooth tile-to-tile movement
• optional creature sprite flipping based on movement direction
• optional animation on visible lower z-levels

Usage

enable smooth-movement
smooth-movement
smooth-movement flip on|off
smooth-movement zlevel on|off

Implementation

The implementation is divided into focused components:

• smooth-movement.cpp: plugin lifecycle, commands, and renderer hook
• visual_animation.cpp/.h: movement detection and animation state
• visual_renderer.cpp/.h: viewport staging and SDL rendering

[notliad]

Its on draft cause I don't know if will require multiple PRs due to its size