Add zstd as a third response encoding alongside gzip and Brotli,
mirrored through the same request-negotiation, config, header, and
static-precompression paths. Priority when several encodings are
acceptable: zstd > br > gzip.

Build:
* CMakeModules/FindZstd.cmake: detect libzstd via pkg-config or
  find_path/find_library, requiring >= 1.4.0 for the streaming API.
* New WITH_ZSTD cache var (AUTO/ON/OFF, default AUTO); unlike
  Brotli's manual toggle, this is a real auto-detect.
* Define USE_ZSTD and link libzstd when found (top-level, src/,
  test/ CMakeLists.txt).

Core:
* util/zstdbuf.{h,cpp}: Compressor backed by libzstd's streaming
  API, mirroring util/brotlibuf.{h,cpp}.
* http/httpreq.{h,cpp}: parse "zstd" from Accept-Encoding; add
  ZSTD_ENABLED/REQ_ZSTD_ACCEPT/ZSTD_REQUIRED/UPSTREAM_ZSTD and
  zstdAcceptable()/andZstd()/orZstd().
* http/httpserverconfig.{h,cpp}: get/setZstdCompress(),
  get/setZstdCompressLevel().
* http/httpvhost.{h,cpp}: VH_ZSTD feature bit, enableZstd(), wired
  to the per-vhost "enableZstd" directive.
* http/httprespheaders.{h,cpp}, httpresp.h: ETAG_ZSTD and
  "content-encoding: zstd" header emission.
* http/staticfilecachedata.{h,cpp}: SFCD_MODE_ZSTD, ".lsq"
  precompressed cache files; refactor tryCreateCompressed/
  compressFile/readyCompressed/compressHelper from a brotli-only
  bool onto an algo enum shared by all three codecs.
* http/staticfilehandler.cpp, sendfileinfo.cpp, subrequest.cpp:
  include zstd in static-file mode selection and cache lookups.
* http/httpsession.cpp: factor zstd into compressibility checks
  and the Content-Encoding strip/passthrough decision. Dynamic
  (on-the-fly) bodies stay gzip-only via modgzip, same as Brotli.
* main/httpserver.cpp, plainconf.cpp: new directives
  enableZstdCompress, zstdCompressLevel, zstdAutoUpdateStatic,
  zstdStaticCompressLevel.
* config.h.in: add USE_ZSTD placeholder.

Tests:
* test/util/zstdbuftest.cpp: streaming round-trip test for
  ZstdBuf, wired into src/ and test/ CMakeLists.txt.