Optional ReadonlybackendDeclared backend label recorded on BvcDecoder.backend. BvcDecoder
always decodes in-process; for the Web Worker pool use BvcWorkerDecoder
(or createBvcDecoder, which selects one). Defaults to 'cpu-wasm'.
Optional Readonlywasmwasm load + init overrides (Node/Bun bytes or a custom URL); see WasmHostInit.
Construction options for BvcDecoder.