Typing conventions for incremental Arthexis adoption

Use these conventions when tightening types in existing modules so MyPy gains signal without forcing a large rewrite.

Preferred shapes

• Model external payloads with TypedDict when the code expects named keys from subprocesses, HTTP APIs, webhooks, or serializer-like data.
• Describe pluggable adapters and callback contracts with Protocol instead of Any or bare duck-typed comments.
• Prefer X | None for nullable values.
• Introduce small module-local type aliases for repeated shapes owned by that module.
• Narrow command and collection inputs to Mapping[str, object], Sequence[str], or similar concrete abstractions when callers do not require mutability.