Add `target_cblock_size` option to `ZstdWriterBase::Options`.

It attempts to fit compressed block size into approximately
`target_cblock_size`. A lower value allows the decompressor to begin
decompression sooner, having less data available, at the cost of
reducing compression density.

This corresponds to `ZSTD_CCtx_setParameter(ZSTD_c_targetCBlockSize)`.

Also, add static assertions about matching constants. The constants cannot be
used in `zstd_writer.h` because they require `ZSTD_STATIC_LINKING_ONLY`.

Internally, store `window_log_` and `target_cblock_size_` without
`std::optional` wrapping, with 0 meaning default. This makes options smaller.

PiperOrigin-RevId: 882591475