feat: add static Go 1.27 simd/archsimd backend (fixes #17)

[hazyhaar]

Description

Fixes #17.

This pull request introduces a native, static SIMD backend for Go 1.27+ (GOEXPERIMENT=simd via simd/archsimd), resolving the fatal initialization crash and enabling base64x in hardened security environments enforcing W^X policies.

Problem Addressed

1. Go 1.27 Crash: In Go 1.27, updated internal runtime symbol tables cause bytedance/sonic/loader (WrapGoC) to abort the process immediately at package initialization (fatal error: invalid function symbol table in runtime.moduledataverify1).
2. W^X / MemoryDenyWriteExecute Incompatibility: Hardened runtime environments (SELinux execmem disabled, systemd MemoryDenyWriteExecute=yes, PaX MPROTECT, iOS) explicitly forbid transitioning memory from writable to executable via mprotect(PROT_READ|PROT_EXEC), causing an unrecoverable panic.

Key Changes

• Go 1.27+ Static Backend (base64x_simd_amd64.go): Conditioned on //go:build goexperiment.simd && amd64 && !noasm && !appengine. Uses pure ahead-of-time Go code with native simd/archsimd intrinsics (Wojciech Muła / Daniel Lemire AVX2 algorithm).
• Pure AVX2 Kernel (internal/simd): Ahead-of-time vector kernel for standard and URL base64 alphabets (with and without padding).
• Zero-Allocation Safety: Handled safe capacity guards for 32-byte chunk writes to prevent memory corruption when destination slices have tight capacities.
• Strict Backward Compatibility:
  • Build tag !goexperiment.simd added to base64x_native.go and base64x_subr_native_amd64.go.
  • Go $\le$ 1.26 continues to run the existing bytecode loader unmodified.
  • Linkname symbols (_subr__b64decode, _subr__b64encode) remain preserved.

Verification & Performance

• Bit-Exact Parity: 100% bit-exact against GCC -O2 -mavx2 (ASAN/UBSAN) on thousands of randomized vectors and differential testing against encoding/base64.
• Passes all tests: Full test suite, CRLF, JSON unescaping, error detection, and fuzzing passes with -race:

  GOEXPERIMENT=simd go test -v -race .
  # PASS: TestUnquoteJSONBase64
  # PASS: TestUnquoteJSONBase64RejectsInvalidEscapes
  # PASS: TestFallbackDecodeMissingPaddingReturnsStdlibError
  # PASS: TestSubrCompatibilitySymbols
  # PASS: TestEncoder
  # PASS: TestDecoder
  # PASS: TestDecoderCRLF
  # PASS: TestDecoderJSON
  # PASS: TestDecoderError
  # PASS: FuzzMain

• Throughput (i9-14900K, 0 B/op, 0 allocs):
  • Encoding: 5.5 to 6.5 GB/s (single-core) / ~19.6 GB/s (parallel 4kB benchmark)
  • Decoding: 3.2 to 3.8 GB/s (single-core) / ~2.4 GB/s (parallel benchmark)

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