v0.3.3

v0.3.3: single-pass struct encoding, deep-nested decode fast path

Performance:
- Single-pass struct encoding pipeline with compile-time derived codegen
- Deep-nested decode fast path: single-entry objects/arrays avoid heap
  allocation entirely (~27% faster for 100-level nested JSON)
- Depth-capped Vec capacity for nested containers
- NIF bypass for iodata Fragments (IO.iodata_to_binary instead of NIF)
- Top-level-only fragment resolution (O(1) instead of O(n))

Cleanup:
- Extract encode! helpers (resolve_protocol_fragments, encode_to_nif)
  to satisfy credo complexity/nesting checks
- Remove jsonrs/benchee dev deps and rustler override
- Gitignore sandbox/
- Alphabetize test aliases
- Update CHANGELOG, README, BENCHMARKS docs

444 tests, 0 failures. All clippy/fmt/credo --strict clean.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>