Feature: no-simd

[Owanesh]

Support for no SIMD environment

• Added RANDOMX_NO_SIMD CMake option to compile WebRandomX with baseline-only WASM instructions (no SIMD128, bulk-memory, sign-extension, non-trapping
  float-to-int). SIMD intrinsics are replaced by scalar equivalents — hash output is bit-identical to the standard build.
• Fixed HEAPU8 is undefined #1: newer Emscripten versions don't expose Module.HEAPU8 by default, breaking wrapper.js at init. Added EXPORTED_RUNTIME_METHODS to link flags.
• Re-enabled auth handshake in miner.js (was commented out, so the server never sent jobs).
• Added NormalModuleReplacementPlugin in webpack config to handle node: prefixed imports from Emscripten output.

What changed

• CMakeLists.txt — RANDOMX_NO_SIMD option, conditional compile/link flags, EXPORTED_RUNTIME_METHODS
• src/cpp/rx_vec_i128.h — new C header: scalar union type + xor/load/store
• src/cpp/intrin_wasm.h — scalar rx_* ops behind #ifdef RANDOMX_NO_SIMD
• src/cpp/intrin_wasm_simd.hpp — scalar shuffle, unpack, mulu
• src/cpp/blamka-round-simd.h — scalar BLAKE2 round (rotr, fBlaMka, G1/G2, DIAG/UNDIAG)
• src/cpp/argon2_simd.c — scalar fill_block using rx_vec_i128
• Makefile.noscimd — build wrapper
• test_simd_comparison.sh — feature audit, correctness, hash equivalence, benchmark
• README_NOSIMD.md — PoC docs
• src/js/miner.js — uncommented auth send
• webpack/webpack.config.common.js — node: prefix plugin

Test plan

• make -f Makefile.noscimd builds clean
• ./test_simd_comparison.sh — all checks pass (identical hashes)
• Standard SIMD build still works