feat: corruption visual progression (tone arc shader)

## Summary
Implement the visual corruption system that drives the tone arc from arcade-bright to Lovecraftian horror.

## Scope
- Corruption stat increases as sectors deepen
- Early: bright flat colors
- Mid: desaturation, color grading
- Late: geometry distortion, horror palette
- Ship visually mutates with corruption level
- Post-process shader pass (optional: 2.5D corner-bend effect noted in DESIGN.md)

## Depends on
- #1 (scaffold)
- #2 (sprites)

Start by reading DESIGN.md for the optional 2.5D corner-bend effect, then review the scaffold from #1 and sprites from #2 before deciding where corruption state and visual progression belong. Done means corruption increases with sector depth, visuals progress from bright colors through grading to distortion and horror palettes, and the ship mutates across corruption levels.

Written by the indexing model from the issue text.