Scaffold Protocols module: abstract types and core structs

[jc-macdonald]

Summary

Port the Python protocols.py type system to Julia using abstract types, structs, and multiple dispatch.

Types to Implement

Python	Julia equivalent
ModelWorld Protocol	abstract type AbstractModelWorld end
Scorer Protocol	abstract type AbstractScorer end
Observable	struct Observable
Tier enum	@enum Tier EMBEDDED PENALIZED DIAGNOSTIC COST
Direction enum	@enum Direction MINIMIZE MAXIMIZE
TrialResult	struct TrialResult
Annotation	struct Annotation
ResultsTable	struct ResultsTable with Tables.jl interface

Design Notes

• Use Julia's native multiple dispatch instead of Protocol ABCs.
• AbstractModelWorld subtypes must implement simulate(::T, params) and observe(::T, sim).
• AbstractScorer subtypes must implement score(::T, obs, pred).
• Consider Tables.jl compliance for ResultsTable.

Acceptance Criteria

• All types exported from TradeStudy
• Docstrings on every exported type
• Smoke tests confirming construction and basic dispatch