Integrate the typed scenario registry into new campaign setup

[aribradshaw]

Player outcome

Let players choose a registered scenario from the new-campaign setup and ensure the selected scenario controls the initial campaign session.

Implementation direction

Follow docs/architecture.md: resolve and validate a ScenarioSession, pass it into engine initialization, apply resources and sparse state overrides before Week 1, and preserve the scenario id and seed in saves.

Acceptance criteria

• The setup screen lists registered scenarios without hard-coded duplicate metadata.
• Invalid scenarios cannot start.
• Save and resume preserve scenario id and deterministic seed.
• Historical 1976 remains the default.
• Unit and browser tests cover selection, initialization, and resume.