Function createBoundedJpegDecoder

Creates a JPEG → RGBA8 decoder whose Wasm memory maximum is set BEFORE any codec code runs. Same contract as createBoundedPngDecoder: the host RGBA copy is independently bounded, a codec failure poisons the instance, and header/budget refusals before codec execution do not.

Pin @jsquash/jpeg@1.6.0's wasm. Native createImageBitmap cannot satisfy maximumWorkingBytes.