Seal pkg — the widest consumer surface #644

Feature: #625 · Part 1, phase 4 of sealed-provider-resources.md.

pkg — 7 files outside its package name the type, the widest surface in part 1, which is why it stands alone.

Answers the plan's standing question: a resource whose exported behavioral fields consumers read must expose them as interface methods, or those consumers change. Size that before transforming it — the answer decides whether this is one PR or several.