M0.2 Benchmark: double-SHA256 throughput (multi-buffer SIMD vs WebCrypto)

[melvincarvalho]

Part of #1 (M0).

Goal

Measure double-SHA256 throughput for txid and merkle hashing across three approaches: naive WASM, multi-buffer SIMD (SHA256 x4/x8), and WebCrypto SubtleCrypto for bulk buffers.

Benchmark / acceptance

• Hashes/sec and MB/s for small (32-80 byte) and large (MBs) inputs.
• Pick the winning approach per size class (small txid hashing vs bulk block hashing).
• Quantify the gap vs native SHA-NI so M0.5 can model it.

Notes

WebCrypto is native/hardware but async with per-call overhead, so likely only wins for large contiguous hashes. Multi-buffer SIMD is the candidate for the billions of tiny hashes.