ci: dedupe round-trip helpers and narrow smoke test scope

Two follow-ups from review on #67:

- Extract shared helpers for the compress/decompress round-trip
  patterns that were copy-pasted across zeroCopyContextCompression,
  streamingZeroCopy, and streamingIo (plain vs. dictionary variants),
  and collapse the repeated check(Arrays.equals(...), msg) pattern
  into a checkArrayEquals helper used throughout.

- Narrow scope: this file's job is proving the released native
  artifact loads and links on this specific OS/arch/libc, and that
  off-heap MemorySegment interop and native struct layouts (streaming
  buffers, frame progression) work there — not re-deriving full API
  correctness, which the module's JUnit suite already covers more
  thoroughly with precise single-platform failures. Dropped the
  parameterBounds()/errorCodeDescriptions() enum sweeps, the 6-way
  dictionary-training-variant sweep, dictionaryIdApi()'s pure-Java
  record assertions, and the redundant dictionary-attachment/overload
  permutations (per-call vs. sticky vs. MemorySegment vs. digested vs.
  refDictionary), keeping one representative path per category. Cuts
  the file from ~610 to ~390 lines.

Compiled and ran locally against zstd-0.7.jar + zstd-native-osx-aarch64-0.7.jar.

Author: dfa1