Review public API mutation boundaries

[rlippmann]

Review whether public API objects expose mutable internals or weaken authoritative state boundaries.

Questions to resolve:

• Should Decision remain a TypedDict-like shape or become a stronger abstraction?
• Should state access return immutable snapshots/copies?
• Can callers mutate returned state accidentally?
• Are Python and TypeScript API boundaries equivalent?

Goal:
Ensure authoritative state changes flow through validated engine transitions.