Export adapter output dataclasses or fix README adapter imports

[laynepenney]

Finding

The README and adapter API docs show customer imports from synapt_eval.adapters that do not work.

Documented examples:

from synapt_eval.adapters import RetrievalAdapter, RetrievalCandidate
from synapt_eval.adapters import GenerationAdapter, GenerationOutput

Actual synapt_eval.adapters.__all__ exports RetrievalAdapter and GenerationAdapter, but not RetrievalCandidate or GenerationOutput.

Repro

Source checkout: b312868.

python -m venv /tmp/synapt-eval-macos311
source /tmp/synapt-eval-macos311/bin/activate
python -m pip install -e ".[dev]"
python - <<'PY'
from synapt_eval.adapters import RetrievalCandidate, GenerationOutput
print(RetrievalCandidate, GenerationOutput)
PY

Observed:

ImportError: cannot import name 'RetrievalCandidate' from 'synapt_eval.adapters'

Expected

Either:

1. export RetrievalCandidate and GenerationOutput from synapt_eval.adapters, matching docs, or
2. update README/docs to use the nested imports that examples currently use:

from synapt_eval.adapters.retrieval_adapter import RetrievalCandidate
from synapt_eval.adapters.generation_adapter import GenerationOutput

Impact

This blocks copy/paste adoption from the README quickstart and adapter docs. The runnable examples work because they use nested imports, but the public docs do not.