Replace jimp with sharp

## Summary

Replace Jimp with [sharp](https://sharp.pixelplumbing.com/) as the image processing library in spring-map-parser.

## Why

| Capability | Jimp | sharp |
|---|---|---|
| **Resampling** | Bilinear only | Lanczos3, Mitchell, cubic, etc. |
| **Large images** | Crashes on 10240×10240 JPEG (jpeg-js RangeError) | Handles any size via libvips streaming |
| **Performance** | Pure JS — slow encode/decode | Native libvips — 10-50× faster |
| **Format support** | JPEG, PNG, BMP, TIFF, GIF | + WebP, AVIF, HEIF |
| **Memory usage** | Full RGBA buffer in JS heap | Streams tiles, constant memory |
| **Alpha handling** | Manual pixel manipulation | Built-in .removeAlpha(), .ensureAlpha() |

## Current Jimp usage in the parser

- 10×
ew Jimp(width, height) — buffer construction
- 1× Jimp.read() — skybox DDS decode
- 2× Jimp.intToRGBA /
gbaToInt — pixel color helpers
- All returned images are Jimp instances with .bitmap.data (raw RGBA Buffer)

All of these have direct sharp equivalents. The main API change is that sharp uses a pipeline pattern (sharp(input).resize().png().toBuffer()) rather than mutable bitmap objects.

## Migration considerations

- **Breaking change**: Consumers currently receive Jimp objects. The return type would become sharp.Sharp or raw Buffer with metadata.
- **Incremental path**: Could first expose .bitmap.data raw buffers (already available on Jimp objects) as the public API, then swap internals to sharp. Downstream consumers like maps-metadata already bridge to sharp via raw buffers.
- **Native dependency**: sharp requires a native binary (pre-built for all major platforms via prebuild). This is standard for Node.js image processing but differs from Jimp's pure-JS approach.
- **DDS decoding**: Jimp.read() is used for skybox DDS files — would need a DDS decoder that outputs raw RGBA for sharp to consume (or keep a minimal DDS reader).

## Related

- beyond-all-reason/maps-metadata#612 — sharp integration in the cloud service (incremental, doesn't require this)
- The cloud service can bridge Jimp→sharp today via raw bitmap data, but full replacement here would simplify the stack