Migrated from spboyer/waza#390

Summary

Design how custom graders beyond config-driven types can be distributed via the registry.

Context

From waza-vs-openai-evals.md Row 15: OpenAI Evals allows Python class inheritance for custom eval logic. Waza is config-driven only (code, program graders). This is waza's #2 competitive gap.

Options

1. WASM plugins — compile grader logic to WASM, distribute via registry. Sandboxed, cross-platform.
2. External program protocol — any binary that accepts stdin JSON and outputs grader results. Already partially supported via program grader.
3. Go plugin interface — plugin.Open() for native Go graders. Fast but platform-specific.
4. Embedded scripting — extend code grader beyond Python (JavaScript, Lua, Starlark).

Parent: #385