WebGPU mining runner.
The implementation wires a compute shader that performs the full
double-SHA256 over tx_prefix || nonce || tx_suffix. The shader mirrors the
CPU implementation in zeldhash-miner-core and returns txids in the same byte order
(big-endian hash bytes, which callers treat as txid by reversing when
counting leading zeros). Storage buffer layouts match the TODO design so the
JavaScript bindings and future optimizations can reuse them unchanged.