Add UI 9-slice sprite kit for redesign (T7)

[Shombith03]

Summary

Implements the component sprite kit for the UI redesign (task T7), delivering 26 white+alpha 9-slice sprites with procedurally-generated PNG assets, metadata, and a test scene. All sprites are authored from a declarative geometry table, enabling verification that shipped assets match the specification exactly.

Key Changes

• Sprite authoring pipeline: Three Python tools that generate and verify the sprite kit:

  • author_ui_sprite_kit.py: Main entry point; generates PNG assets and .meta files from the KIT table
  • ui_sprite_kit_geometry.py: Pure-stdlib geometry engine and PNG codec; computes corner slivers (45° diagonal cuts on opposite corners per Style Foundation §5) and encodes RGBA pixels
  • verify_ui_sprite_kit.py: Post-generation verification; proves each PNG is white+alpha, validates spriteBorder metadata, and confirms partially-transparent pixels lie strictly inside border corners (the invariant that makes 9-slice scaling preserve diagonals)

• 26 generated sprite assets under Assets/_Graphics/UI/SpriteKit/:

  • Banner, Button, ButtonSmall, Card, CurrencyPill, HexHandle, HexTile, Panel families
  • Fill and Border variants; Border variants include flipped versions for corner sliver orientation
  • All RGB 255,255,255 with shape carried entirely in alpha (enables runtime tinting from UIThemeSO)

• Test scene Assets/_Scenes/Game_TestDesign/UISpriteKitTestScene.unity:

  • One full-screen page per sprite family; page 1 active, rest inactive (toggle in Hierarchy)
  • Each row displays one sprite at three widths (plus fourth sample at double height for vertical-border sprites)
  • Each sample tinted a different Style Foundation Sec.2 colour to demonstrate the asset carries no colour

• Documentation Docs/UI_SPRITE_KIT.md: Overview, usage, and verification instructions

Implementation Details

• No hand-edited pixels or YAML: Everything derives from the KIT table in author_ui_sprite_kit.py, so --check mode can prove committed files are exactly what the table specifies
• Coordinate system: Pixel space, x right, y DOWN from top-left (matching PNG row order); Unity sprite borders are {x=left, y=bottom, z=right, w=top}
• Corner sliver geometry: 45° diagonal cuts on TWO OPPOSITE corners per Style Foundation v0.3.1 §5 (supersedes earlier single-chamfer shapes in shipped Button_Flat_*.png)
• No new C# code: Scene is entirely declarative UGUI, keeping it outside the /verify-unity compile gate
• Standard library only: Authoring tools use only Python stdlib for reproducibility across build machines, CI, and designer laptops

Spec References

• Style Foundation §5 (geometry) and §10 (components)
• Docs/UI_REDESIGN_TASKS.md task T7

https://claude.ai/code/session_01H4Lq17kBtCJ2LbQGQ8ikfw