Add XTCH 2-bit conversion support

[varo6]

Summary

• add XTCH as a basic conversion option and emit true XTH page payloads inside XTCH containers
• extend the dithering and worker pipelines to quantize to 4 grayscale levels for 2-bit output
• preserve XTCH files in merge/split flows and update app copy to mention XTC/XTCH support

Verification

• bun run build
• bun -e "import { imageDataToXth } from './src/lib/processing/xtg.ts'; import { buildXtcFromBuffers } from './src/lib/xtc-format.ts'; import { parseXtcFile } from './src/lib/xtc-reader.ts'; const width=8, height=8; const data=new Uint8ClampedArray(widthheight4); for (let y=0; y<height; y++) { for (let x=0; x<width; x++) { const idx=(y*width+x)*4; const val=((x+y)%4)*85; data[idx]=data[idx+1]=data[idx+2]=val; data[idx+3]=255; } } const page=imageDataToXth({ width, height, data }); const xtch=await buildXtcFromBuffers([page], { is2bit: true }); const parsed=await parseXtcFile(xtch); console.log({ pageMagic:String.fromCharCode(...new Uint8Array(page).slice(0,3)), is2bit:parsed.header.is2bit, payloadMagic:String.fromCharCode(...new Uint8Array(parsed.pageData[0]).slice(0,3)) });"