v0.4.0

v0.4.0: Return {:ok, ctx} | {:error, reason} from context constructors

Breaking API change: create_compression_context/0 and
create_decompression_context/0 now return ok/error tuples instead of
bare references, matching the rest of the API.

- Make C++ constructors noexcept (no more throws from NIF land)
- Propagate ZL_CCtx_setParameter errors in factory NIFs
- Add null check to create_sddl_compressor after noexcept constructor
- Add thread-safety documentation to moduledoc and context functions
- Update README: fix dep version, add precompiled binaries section,
  update examples to {:ok, ctx} pattern, add thread-safety note
- Add CI workflow (.github/workflows/ci.yml)
- Bump version to 0.4.0

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>