[Perf][C++] SIMD bit-unpack kernels run as portable fallbacks in default builds; add runtime AVX2 dispatch #1073

Problem

ENABLE_SIMD only defines -DENABLE_SIMD. The library targets the portable x86-64 baseline (SSE2) and never compiles with -mavx2/-mssse3, so simde's AVX2/SSSE3 intrinsics (vpgatherqq, vpshufb, ...) compile to simde's portable fallback.

Micro-benchmark of the RLE bit-unpack (same header, only difference is -mavx2):

• width 14: 99 → 644 M values/s (6.5×)
• width 20: 99 → 612 M values/s (6.2×)

So the SIMD paths added in #823 / #898 (PLAIN, GORILLA, TS_2DIFF) and the RLE packers were not running real AVX2 in shipped builds.

Proposed changes

• Add common/simd_dispatch.h with cpu_has_avx2() (__builtin_cpu_supports("avx2"), GCC/Clang on x86 only).
• Add __attribute__((target("avx2"))) intrinsic twins of the bit-unpack kernels, dispatched at runtime; keep the simde path as the portable fallback.

Implemented for RLE and TS_2DIFF in #1072. PLAIN / GORILLA kernels can follow the same pattern.

Related: #1072